Index: sage/rings/polynomial/multi_polynomial_libsingular.pyx
===================================================================
--- sage/rings/polynomial/multi_polynomial_libsingular.pyx	(revision 7809)
+++ sage/rings/polynomial/multi_polynomial_libsingular.pyx	(revision 7987)
@@ -1514,4 +1514,7 @@
         cdef number *h
         cdef int ret = 0
+
+        if left is right:
+            return ret
         
         r = (<MPolynomialRing_libsingular>left._parent)._ring