diff --git a/rebase.go b/rebase.go
index aa34c9e..2144329 100644
--- a/rebase.go
+++ b/rebase.go
@@ -98,7 +98,14 @@ func commitSignCallback(_signature *C.git_buf, _signature_field *C.git_buf, _com
 		cstr := unsafe.Pointer(C.CString(bufData))
 		defer C.free(cstr)
 
-		if int(C.git_buf_set(buf, cstr, clen)) != 0 {
+		// over-assign by a byte (see below)
+		if int(C.git_buf_grow(buf, clen+1)) != 0 {
+			return errors.New("could not grow buffer")
+		}
+
+		C._go_git_buf_fill_null(buf)
+
+		if int(C.git_buf_set(buf, cstr, clen+1)) != 0 {
 			return errors.New("could not set buffer")
 		}
 
diff --git a/wrapper.c b/wrapper.c
index 90b0e1e..6bdc11a 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -12,6 +12,11 @@ void _go_git_populate_apply_cb(git_apply_options *options)
 	options->hunk_cb = (git_apply_hunk_cb)hunkApplyCallback;
 }
 
+void _go_git_buf_fill_null(git_buf *buf)
+{
+  memset(buf->ptr, '\0', buf->asize*sizeof(char));
+}
+
 void _go_git_populate_commit_sign_cb(git_rebase_options *opts)
 {
 	opts->signing_cb = (git_commit_signing_cb)commitSignCallback;