diff --git a/collects/mzlib/private/contract.ss b/collects/mzlib/private/contract.ss
index 1d1db452fd..dd9830dec0 100644
--- a/collects/mzlib/private/contract.ss
+++ b/collects/mzlib/private/contract.ss
@@ -1080,10 +1080,24 @@ add struct contracts for immutable structs?
                     (λ (x) 
                       (and (number? x)
                            (<= n x m))))))))
-  (define (=/c x) (make-between/c x x))
-  (define (<=/c x) (make-between/c -inf.0 x))
-  (define (>=/c x) (make-between/c x +inf.0))
-  (define (between/c x y) (make-between/c x y))
+  (define (=/c x) 
+    (unless (number? x)
+      (error '=/c "expected a number, got ~e" x))
+    (make-between/c x x))
+  (define (<=/c x) 
+    (unless (number? x)
+      (error '<=/c "expected a number, got ~e" x))
+    (make-between/c -inf.0 x))
+  (define (>=/c x)
+    (unless (number? x)
+      (error '>=/c "expected a number, got ~e" x))
+    (make-between/c x +inf.0))
+  (define (between/c x y)
+    (unless (number? x)
+      (error 'between/c "expected a number as first argument, got ~e, other arg ~e" x y))
+    (unless (number? y)
+      (error 'between/c "expected a number as second argument, got ~e, other arg ~e" y x))
+    (make-between/c x y))
 
   (define (</c x)
     (flat-named-contract