diff --git a/checkout.go b/checkout.go
index f4c1d4e..d3cd47b 100644
--- a/checkout.go
+++ b/checkout.go
@@ -65,7 +65,7 @@ func (v *Repository) Checkout(opts *CheckoutOpts) error {
 
 	ret := C.git_checkout_head(v.ptr, &copts)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 
 	return nil
@@ -81,7 +81,7 @@ func (v *Repository) CheckoutIndex(index *Index, opts *CheckoutOpts) error {
 
 	ret := C.git_checkout_index(v.ptr, index.ptr, &copts)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 
 	return nil
diff --git a/commit.go b/commit.go
index 0c64c76..0edebb6 100644
--- a/commit.go
+++ b/commit.go
@@ -31,7 +31,7 @@ func (c Commit) Tree() (*Tree, error) {
 
 	err := C.git_commit_tree(&ptr, c.ptr)
 	if err < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(err)
 	}
 
 	return allocObject(ptr).(*Tree), nil
diff --git a/config.go b/config.go
index 7665b20..2234a43 100644
--- a/config.go
+++ b/config.go
@@ -94,7 +94,7 @@ func (c *Config) LookupInt32(name string) (int32, error) {
 
 	ret := C.git_config_get_int32(&out, c.ptr, cname)
 	if ret < 0 {
-		return 0, LastError()
+		return 0, MakeGitError(ret)
 	}
 
 	return int32(out), nil
@@ -110,7 +110,7 @@ func (c *Config) LookupInt64(name string) (int64, error) {
 
 	ret := C.git_config_get_int64(&out, c.ptr, cname)
 	if ret < 0 {
-		return 0, LastError()
+		return 0, MakeGitError(ret)
 	}
 
 	return int64(out), nil
@@ -125,7 +125,7 @@ func (c *Config) LookupString(name string) (string, error) {
 	defer runtime.UnlockOSThread()
 
 	if ret := C.git_config_get_string(&ptr, c.ptr, cname); ret < 0 {
-		return "", LastError()
+		return "", MakeGitError(ret)
 	}
 
 	return C.GoString(ptr), nil
@@ -220,7 +220,7 @@ func (c *Config) SetString(name, value string) (err error) {
 
 	ret := C.git_config_set_string(c.ptr, cname, cvalue)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 
 	return nil
diff --git a/git.go b/git.go
index 72ea33e..7c76c30 100644
--- a/git.go
+++ b/git.go
@@ -61,8 +61,8 @@ func NewOidFromString(s string) (*Oid, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	if C.git_oid_fromstr(o.toC(), cs) < 0 {
-		return nil, LastError()
+	if ret := C.git_oid_fromstr(o.toC(), cs); ret < 0 {
+		return nil, MakeGitError(ret)
 	}
 
 	return o, nil
@@ -123,27 +123,36 @@ func ShortenOids(ids []*Oid, minlen int) (int, error) {
 		buf[40] = 0
 		ret = C.git_oid_shorten_add(shorten, (*C.char)(unsafe.Pointer(&buf[0])))
 		if ret < 0 {
-			return int(ret), LastError()
+			return int(ret), MakeGitError(ret)
 		}
 	}
 	return int(ret), nil
 }
 
 type GitError struct {
-	Message string
-	Code    int
+	Message   string
+	Class     int
+	ErrorCode int
 }
 
 func (e GitError) Error() string {
 	return e.Message
 }
 
-func LastError() error {
+func IsNotExist(err error) bool {
+	return err.(*GitError).ErrorCode == C.GIT_ENOTFOUND
+}
+
+func IsExist(err error) bool {
+	return err.(*GitError).ErrorCode == C.GIT_EEXISTS
+}
+
+func MakeGitError(errorCode C.int) error {
 	err := C.giterr_last()
 	if err == nil {
-		return &GitError{"No message", 0}
+		return &GitError{"No message", C.GITERR_INVALID, C.GIT_ERROR}
 	}
-	return &GitError{C.GoString(err.message), int(err.klass)}
+	return &GitError{C.GoString(err.message), int(err.klass), int(errorCode)}
 }
 
 func cbool(b bool) C.int {
@@ -175,7 +184,7 @@ func Discover(start string, across_fs bool, ceiling_dirs []string) (string, erro
 
 	ret := C.git_repository_discover(&buf, cstart, cbool(across_fs), ceildirs)
 	if ret < 0 {
-		return "", LastError()
+		return "", MakeGitError(ret)
 	}
 
 	return C.GoString(buf.ptr), nil
diff --git a/index.go b/index.go
index bc11025..cbb28c4 100644
--- a/index.go
+++ b/index.go
@@ -42,7 +42,7 @@ func (v *Index) AddByPath(path string) error {
 
 	ret := C.git_index_add_bypath(v.ptr, cstr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 
 	return nil
@@ -56,7 +56,7 @@ func (v *Index) WriteTree() (*Oid, error) {
 
 	ret := C.git_index_write_tree(oid.toC(), v.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return oid, nil
diff --git a/odb.go b/odb.go
index 953791c..3daf3a4 100644
--- a/odb.go
+++ b/odb.go
@@ -32,7 +32,7 @@ func (v *Odb) Write(data []byte, otype ObjectType) (oid *Oid, err error) {
 	ret := C.git_odb_write(oid.toC(), v.ptr, unsafe.Pointer(hdr.Data), C.size_t(hdr.Len), C.git_otype(otype))
 
 	if ret < 0 {
-		err = LastError()
+		err = MakeGitError(ret)
 	}
 
 	return
@@ -46,7 +46,7 @@ func (v *Odb) Read(oid *Oid) (obj *OdbObject, err error) {
 
 	ret := C.git_odb_read(&obj.ptr, v.ptr, oid.toC())
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	runtime.SetFinalizer(obj, (*OdbObject).Free)
diff --git a/packbuilder.go b/packbuilder.go
index 333f183..70c4530 100644
--- a/packbuilder.go
+++ b/packbuilder.go
@@ -28,7 +28,7 @@ func (repo *Repository) NewPackbuilder() (*Packbuilder, error) {
 
 	ret := C.git_packbuilder_new(&builder.ptr, repo.ptr)
 	if ret != 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 	runtime.SetFinalizer(builder, (*Packbuilder).Free)
 	return builder, nil
@@ -48,7 +48,7 @@ func (pb *Packbuilder) Insert(id *Oid, name string) error {
 
 	ret := C.git_packbuilder_insert(pb.ptr, id.toC(), cname)
 	if ret != 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -59,7 +59,7 @@ func (pb *Packbuilder) InsertCommit(id *Oid) error {
 
 	ret := C.git_packbuilder_insert_commit(pb.ptr, id.toC())
 	if ret != 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -70,7 +70,7 @@ func (pb *Packbuilder) InsertTree(id *Oid) error {
 
 	ret := C.git_packbuilder_insert_tree(pb.ptr, id.toC())
 	if ret != 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -88,7 +88,7 @@ func (pb *Packbuilder) WriteToFile(name string, mode os.FileMode) error {
 
 	ret := C.git_packbuilder_write(pb.ptr, cname, C.uint(mode.Perm()), nil, nil)
 	if ret != 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
diff --git a/reference.go b/reference.go
index a2f1636..39d328e 100644
--- a/reference.go
+++ b/reference.go
@@ -45,7 +45,7 @@ func (v *Reference) SetSymbolicTarget(target string, sig *Signature, msg string)
 
 	ret := C.git_reference_symbolic_set_target(&ptr, v.ptr, ctarget, csig, cmsg)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -65,7 +65,7 @@ func (v *Reference) SetTarget(target *Oid, sig *Signature, msg string) (*Referen
 
 	ret := C.git_reference_set_target(&ptr, v.ptr, target.toC(), csig, cmsg)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -79,7 +79,7 @@ func (v *Reference) Resolve() (*Reference, error) {
 
 	ret := C.git_reference_resolve(&ptr, v.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -102,7 +102,7 @@ func (v *Reference) Rename(name string, force bool, sig *Signature, msg string)
 	ret := C.git_reference_rename(&ptr, v.ptr, cname, cbool(force), csig, cmsg)
 
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -128,7 +128,7 @@ func (v *Reference) Delete() error {
 	ret := C.git_reference_delete(v.ptr)
 
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 
 	return nil
@@ -173,7 +173,7 @@ func (repo *Repository) NewReferenceIterator() (*ReferenceIterator, error) {
 
 	ret := C.git_reference_iterator_new(&ptr, repo.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	iter := &ReferenceIterator{repo: repo, ptr: ptr}
@@ -194,7 +194,7 @@ func (repo *Repository) NewReferenceIteratorGlob(glob string) (*ReferenceIterato
 
 	ret := C.git_reference_iterator_glob_new(&ptr, repo.ptr, cstr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	iter := &ReferenceIterator{repo: repo, ptr: ptr}
@@ -215,7 +215,7 @@ func (v *ReferenceIterator) NextName() (string, error) {
 		return "", ErrIterOver
 	}
 	if ret < 0 {
-		return "", LastError()
+		return "", MakeGitError(ret)
 	}
 
 	return C.GoString(ptr), nil
diff --git a/repository.go b/repository.go
index 1d58d7e..e78422e 100644
--- a/repository.go
+++ b/repository.go
@@ -26,7 +26,7 @@ func OpenRepository(path string) (*Repository, error) {
 
 	ret := C.git_repository_open(&repo.ptr, cpath)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	runtime.SetFinalizer(repo, (*Repository).Free)
@@ -44,7 +44,7 @@ func InitRepository(path string, isbare bool) (*Repository, error) {
 
 	ret := C.git_repository_init(&repo.ptr, cpath, ucbool(isbare))
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	runtime.SetFinalizer(repo, (*Repository).Free)
@@ -64,7 +64,7 @@ func (v *Repository) Config() (*Config, error) {
 
 	ret := C.git_repository_config(&config.ptr, v.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	runtime.SetFinalizer(config, (*Config).Free)
@@ -79,7 +79,7 @@ func (v *Repository) Index() (*Index, error) {
 
 	ret := C.git_repository_index(&ptr, v.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return newIndexFromC(ptr), nil
@@ -93,7 +93,7 @@ func (v *Repository) lookupType(id *Oid, t ObjectType) (Object, error) {
 
 	ret := C.git_object_lookup(&ptr, v.ptr, id.toC(), C.git_otype(t))
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return allocObject(ptr), nil
@@ -140,7 +140,7 @@ func (v *Repository) LookupReference(name string) (*Reference, error) {
 
 	ecode := C.git_reference_lookup(&ptr, v.ptr, cname)
 	if ecode < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ecode)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -163,7 +163,7 @@ func (v *Repository) CreateReference(name string, id *Oid, force bool, sig *Sign
 
 	ecode := C.git_reference_create(&ptr, v.ptr, cname, id.toC(), cbool(force), csig, cmsg)
 	if ecode < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ecode)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -189,7 +189,7 @@ func (v *Repository) CreateSymbolicReference(name, target string, force bool, si
 
 	ecode := C.git_reference_symbolic_create(&ptr, v.ptr, cname, ctarget, cbool(force), csig, cmsg)
 	if ecode < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ecode)
 	}
 
 	return newReferenceFromC(ptr), nil
@@ -203,7 +203,7 @@ func (v *Repository) Walk() (*RevWalk, error) {
 
 	ecode := C.git_revwalk_new(&walk.ptr, v.ptr)
 	if ecode < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ecode)
 	}
 
 	walk.repo = v
@@ -250,7 +250,7 @@ func (v *Repository) CreateCommit(
 		nil, cmsg, tree.ptr, C.size_t(nparents), parentsarg)
 
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return oid, nil
@@ -268,7 +268,7 @@ func (v *Repository) Odb() (odb *Odb, err error) {
 	defer runtime.UnlockOSThread()
 
 	if ret := C.git_repository_odb(&odb.ptr, v.ptr); ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	runtime.SetFinalizer(odb, (*Odb).Free)
@@ -294,9 +294,10 @@ func (repo *Repository) SetWorkdir(workdir string, updateGitlink bool) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	if C.git_repository_set_workdir(repo.ptr, cstr, cbool(updateGitlink)) < 0 {
-		return LastError()
+	if ret := C.git_repository_set_workdir(repo.ptr, cstr, cbool(updateGitlink)); ret < 0 {
+		return MakeGitError(ret)
 	}
+
 	return nil
 }
 
@@ -307,7 +308,7 @@ func (v *Repository) TreeBuilder() (*TreeBuilder, error) {
 	defer runtime.UnlockOSThread()
 
 	if ret := C.git_treebuilder_create(&bld.ptr, nil); ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 	runtime.SetFinalizer(bld, (*TreeBuilder).Free)
 
@@ -326,7 +327,7 @@ func (v *Repository) RevparseSingle(spec string) (Object, error) {
 
 	ecode := C.git_revparse_single(&ptr, v.ptr, cspec)
 	if ecode < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ecode)
 	}
 
 	return allocObject(ptr), nil
diff --git a/submodule.go b/submodule.go
index 9abf333..8390e36 100644
--- a/submodule.go
+++ b/submodule.go
@@ -81,7 +81,7 @@ func (repo *Repository) LookupSubmodule(name string) (*Submodule, error) {
 
 	ret := C.git_submodule_lookup(&sub.ptr, repo.ptr, cname)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 
 	return sub, nil
@@ -102,7 +102,7 @@ func (repo *Repository) ForeachSubmodule(cbk SubmoduleCbk) error {
 
 	ret := C._go_git_visit_submodule(repo.ptr, unsafe.Pointer(&cbk))
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -120,7 +120,7 @@ func (repo *Repository) AddSubmodule(url, path string, use_git_link bool) (*Subm
 
 	ret := C.git_submodule_add_setup(&sub.ptr, repo.ptr, curl, cpath, cbool(use_git_link))
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 	return sub, nil
 }
@@ -131,7 +131,7 @@ func (sub *Submodule) FinalizeAdd() error {
 
 	ret := C.git_submodule_add_finalize(sub.ptr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -142,7 +142,7 @@ func (sub *Submodule) AddToIndex(write_index bool) error {
 
 	ret := C.git_submodule_add_to_index(sub.ptr, cbool(write_index))
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -153,7 +153,7 @@ func (sub *Submodule) Save() error {
 
 	ret := C.git_submodule_save(sub.ptr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -188,7 +188,7 @@ func (sub *Submodule) SetUrl(url string) error {
 
 	ret := C.git_submodule_set_url(sub.ptr, curl)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -247,7 +247,7 @@ func (sub *Submodule) SetFetchRecurseSubmodules(recurse SubmoduleRecurse) error
 
 	ret := C.git_submodule_set_fetch_recurse_submodules(sub.ptr, C.git_submodule_recurse_t(recurse))
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -258,7 +258,7 @@ func (sub *Submodule) Init(overwrite bool) error {
 
 	ret := C.git_submodule_init(sub.ptr, cbool(overwrite))
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -269,7 +269,7 @@ func (sub *Submodule) Sync() error {
 
 	ret := C.git_submodule_sync(sub.ptr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -282,7 +282,7 @@ func (sub *Submodule) Open() (*Repository, error) {
 
 	ret := C.git_submodule_open(&repo.ptr, sub.ptr)
 	if ret < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(ret)
 	}
 	return repo, nil
 }
@@ -293,7 +293,7 @@ func (sub *Submodule) Reload() error {
 
 	ret := C.git_submodule_reload(sub.ptr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
@@ -304,7 +304,7 @@ func (repo *Repository) ReloadAllSubmodules() error {
 
 	ret := C.git_submodule_reload_all(repo.ptr)
 	if ret < 0 {
-		return LastError()
+		return MakeGitError(ret)
 	}
 	return nil
 }
diff --git a/tree.go b/tree.go
index 8c74e5d..4efb589 100644
--- a/tree.go
+++ b/tree.go
@@ -109,7 +109,7 @@ func (t Tree) Walk(callback TreeWalkCallback) error {
 	)
 
 	if err < 0 {
-		return LastError()
+		return MakeGitError(err)
 	}
 
 	return nil
@@ -134,7 +134,7 @@ func (v *TreeBuilder) Insert(filename string, id *Oid, filemode int) (error) {
 
 	err := C.git_treebuilder_insert(nil, v.ptr, cfilename, id.toC(), C.git_filemode_t(filemode))
 	if err < 0 {
-		return LastError()
+		return MakeGitError(err)
 	}
 
 	return nil
@@ -149,7 +149,7 @@ func (v *TreeBuilder) Write() (*Oid, error) {
 	err := C.git_treebuilder_write(oid.toC(), v.repo.ptr, v.ptr)
 
 	if err < 0 {
-		return nil, LastError()
+		return nil, MakeGitError(err)
 	}
 
 	return oid, nil
diff --git a/walk.go b/walk.go
index 619188d..cdc1a20 100644
--- a/walk.go
+++ b/walk.go
@@ -40,7 +40,7 @@ func (v *RevWalk) PushHead() (err error) {
 
 	ecode := C.git_revwalk_push_head(v.ptr)
 	if ecode < 0 {
-		err = LastError()
+		err = MakeGitError(ecode)
 	}
 
 	return
@@ -55,7 +55,7 @@ func (v *RevWalk) Next(id *Oid) (err error) {
 	case ret == ITEROVER:
 		err = io.EOF
 	case ret < 0:
-		err = LastError()
+		err = MakeGitError(ret)
 	}
 
 	return