diff --git a/pl/arith.yap b/pl/arith.yap
index c4438d7de..e788c060d 100644
--- a/pl/arith.yap
+++ b/pl/arith.yap
@@ -337,7 +337,9 @@ expand_expr(Op, X, Y, O, Q, P) :-
 	'$do_and'(Z = X, Y = W, E).
 
 
-'$goal_expansion_allowed'(phrase(NT,_Xs0,_Xs), Mod).
+'$goal_expansion_allowed'(phrase(NT,_Xs0,_Xs), Mod) :-
+    callable(NT),
+    atom(Mod).
 
 %%	contains_illegal_dcgnt(+Term) is semidet.
 %