diff --git a/Source/NSBezierPath.m b/Source/NSBezierPath.m
index 888b0f8b3..5d79eeadd 100644
--- a/Source/NSBezierPath.m
+++ b/Source/NSBezierPath.m
@@ -1622,6 +1622,13 @@ typedef struct _PathElement
   PathElement elem;
   int i, count;
 
+  if (![aPath isKindOfClass: isa])
+    {
+      [super appendBezierPath: aPath];
+      return;
+    }
+
+  flat = flat && ((GSBezierPath*)aPath)->flat;
   count = [aPath elementCount];
 
   for (i = 0; i < count; i++)