diff --git a/pkgs/racket-test-core/tests/racket/optimize.rktl b/pkgs/racket-test-core/tests/racket/optimize.rktl
index 5730c39785..1da94bca11 100644
--- a/pkgs/racket-test-core/tests/racket/optimize.rktl
+++ b/pkgs/racket-test-core/tests/racket/optimize.rktl
@@ -1356,7 +1356,10 @@
            '(lambda ()
               (let ([y (random)])
                 (begin0 y (set! y 5)))))
-                
+
+(test-comp '(lambda (w) (car w) (mcar w))
+           '(lambda (w) (begin (car w) (mcar w) (random))))
+
 ; test for unary aplications
 (test-comp -1
            '(- 1))
diff --git a/racket/src/racket/src/optimize.c b/racket/src/racket/src/optimize.c
index 41d3c683d7..212934fcd7 100644
--- a/racket/src/racket/src/optimize.c
+++ b/racket/src/racket/src/optimize.c
@@ -2840,7 +2840,8 @@ static void check_known2(Optimize_Info *info, Scheme_App2_Rec *app,
                          Scheme_Object *rand, int id_offset,
                          const char *who, Scheme_Object *expect_pred, Scheme_Object *unsafe)
 /* Replace the rator with an unsafe version if we know that it's ok. Alternatively,
-   the rator implies a check, so add type information for subsequent expressions. */
+   the rator implies a check, so add type information for subsequent expressions. 
+   If the rand has alredy a different type, mark that this will generate an error. */
 {
   if (IS_NAMED_PRIM(app->rator, who)) {
     if (SAME_TYPE(SCHEME_TYPE(rand), scheme_local_type)) {
@@ -2853,8 +2854,11 @@ static void check_known2(Optimize_Info *info, Scheme_App2_Rec *app,
           return;
 
         pred = optimize_get_predicate(pos, info);
-        if (pred && SAME_OBJ(pred, expect_pred))
-          app->rator = unsafe;
+        if (pred)
+          if (SAME_OBJ(pred, expect_pred))
+            app->rator = unsafe;
+          else
+            info->escapes = 1;
         else
           add_type(info, pos, expect_pred);
       }