diff --git a/vcg/simplex/face/pos.h b/vcg/simplex/face/pos.h
index 8c71f1a5..a4047184 100644
--- a/vcg/simplex/face/pos.h
+++ b/vcg/simplex/face/pos.h
@@ -190,10 +190,11 @@ public:
 	void FlipF()
 	{
 		assert( f->FFp(z)->FFp(f->FFi(z))==f );  // two manifoldness check
+		// Check that pos vertex is one of the current z-th edge and it is different from the vert opposite to the edge.
 		assert(f->V(f->Prev(z))!=v && (f->V(f->Next(z))==v || f->V((z))==v));
 		FaceType *nf=f->FFp(z);
 		int nz=f->FFi(z);
-		assert(nf->V(f->Prev(nz))!=v && (nf->V(f->Next(nz))==v || nf->V((nz))==v));
+		assert(nf->V(nf->Prev(nz))!=v && (nf->V(nf->Next(nz))==v || nf->V((nz))==v));
 		f=nf;
 		z=nz;
 		assert(f->V(f->Prev(z))!=v && (f->V(f->Next(z))==v || f->V(z)==v));