diff --git a/diff.go b/diff.go
index 09481b4..6f3f554 100644
--- a/diff.go
+++ b/diff.go
@@ -379,11 +379,11 @@ func (v *Repository) DiffTreeToTree(oldTree, newTree *Tree, opts *DiffOptions) (
 	var oldPtr, newPtr *C.git_tree
 
 	if oldTree != nil {
-		oldPtr = oldTree.gitObject.ptr
+		oldPtr = oldTree.cast_ptr
 	}
 
 	if newTree != nil {
-		newPtr = newTree.gitObject.ptr
+		newPtr = newTree.cast_ptr
 	}
 
 	cpathspec := C.git_strarray{}