diff --git a/src/lib/arithmetic.pl b/src/lib/arithmetic.pl
index 3d3a6625..dc9ce077 100644
--- a/src/lib/arithmetic.pl
+++ b/src/lib/arithmetic.pl
@@ -123,5 +123,5 @@ rational_numerator_denominator(R, N, D) :-
     number_chars(D, Ds).
 
 popcount(X, N) :-
-    integer(X),
+    must_be(integer, X),
     '$popcount'(X, N).