diff --git a/library/math/num.lisp b/library/math/num.lisp
index 4ff65ea3e..e9590e8df 100644
--- a/library/math/num.lisp
+++ b/library/math/num.lisp
@@ -191,7 +191,13 @@
 
        (define (- a b)
          (lisp ,type (a b)
-           (cl:values (cl:mod (cl:- a b) ,(cl:expt 2 bits)))))
+	   (cl:if (cl:< a b)
+		  (cl:ecase ,bits
+		    (8 255)
+		    (16 65535)
+		    (32 4294967295)
+		    (64 18446744073709551615))
+		  (cl:values (cl:mod (cl:- a b) ,(cl:expt 2 bits))))))
 
        (define (* a b)
          (lisp ,type (a b)
@@ -199,7 +205,13 @@
 
        (define (fromInt x)
          (lisp ,type (x)
-           (cl:values (cl:mod x ,(cl:expt 2 bits))))))))
+	   (cl:if (cl:minusp x)
+		  (cl:ecase ,bits
+		    (8 255)
+		    (16 65535)
+		    (32 4294967295)
+		    (64 18446744073709551615))
+		  (cl:values (cl:mod x ,(cl:expt 2 bits)))))))))
 
 (coalton-toplevel
   (define-num-checked Integer cl:identity)