--- src/templates/object.derived
+++ src/templates/object.derived
@@ -149,8 +149,16 @@
     if (impl != null) {
       PyObject res = impl.__get__(this,self_type).__call__();
       if (res instanceof PyInteger)
-         return ((PyInteger)res).getValue();
-      throw Py.TypeError("__len__ should return a int");
+        return ((PyInteger)res).getValue();
+      if (impl instanceof PyInstance) {
+        throw Py.TypeError("__len__ should return a int");
+      } else {
+        if (res instanceof PyLong) {
+          throw Py.OverflowError("long int too large to convert to int");
+        } else {
+          throw Py.TypeError("an integer is required");
+        }
+      }
     }
     return super.__len__();
   }