diff --git a/compiler/types.nim b/compiler/types.nim
index 7b59fbf200..7a9b6bb85f 100644
--- a/compiler/types.nim
+++ b/compiler/types.nim
@@ -969,6 +969,7 @@ proc inheritanceDiff*(a, b: PType): int =
   # | returns: -x iff `a` is the x'th direct superclass of `b`
   # | returns: +x iff `a` is the x'th direct subclass of `b`
   # | returns: `maxint` iff `a` and `b` are not compatible at all
+  if a == b or a.kind == tyError or b.kind == tyError: return 0
   assert a.kind == tyObject
   assert b.kind == tyObject
   var x = a