diff --git a/racket/src/cs/rumble/string.ss b/racket/src/cs/rumble/string.ss
index 53441a12c0..dbf767d595 100644
--- a/racket/src/cs/rumble/string.ss
+++ b/racket/src/cs/rumble/string.ss
@@ -17,17 +17,30 @@
    [(dest d-start src s-start)
     (string-copy! dest d-start src s-start (and (string? src) (string-length src)))]
    [(dest d-start src s-start s-end)
-    (check who mutable-string? :contract "(and/c string? (not/c immutable?))" dest)
-    (check who exact-nonnegative-integer? d-start)
-    (check who string? src)
-    (check who exact-nonnegative-integer? s-start)
-    (check who exact-nonnegative-integer? s-end)
-    (let ([d-len (string-length dest)])
-      (check-range who "string" dest d-start #f d-len)
-      (check-range who "string" src s-start s-end (string-length src))
-      (let ([s-len (fx- s-end s-start)])
-        (check-space who "string" d-start d-len s-len)
-        (#%string-copy! src s-start dest d-start s-len)))]))
+    ;; start with fast, inlined checks for valid calls, then use
+    ;; slower tests with consistent reporting if fast tests fail
+    (cond
+     [(and (mutable-string? dest)
+           (string? src)
+           (fixnum? d-start)
+           (fixnum? s-start)
+           (fixnum? s-end)
+           (fx- s-end s-start)
+           (fx<= 0 d-start (fx+ d-start (fx- s-end s-start)) (string-length dest))
+           (fx<= 0 s-start s-end (string-length src)))
+      (#%string-copy! src s-start dest d-start  (fx- s-end s-start))]
+     [else
+      (check who mutable-string? :contract "(and/c string? (not/c immutable?))" dest)
+      (check who exact-nonnegative-integer? d-start)
+      (check who string? src)
+      (check who exact-nonnegative-integer? s-start)
+      (check who exact-nonnegative-integer? s-end)
+      (let ([d-len (string-length dest)])
+        (check-range who "string" dest d-start #f d-len)
+        (check-range who "string" src s-start s-end (string-length src))
+        (let ([s-len (fx- s-end s-start)])
+          (check-space who "string" d-start d-len s-len)
+          (#%string-copy! src s-start dest d-start s-len)))])]))
 
 (define/who substring
   (case-lambda