diff --git a/pkgs/racket-test-core/tests/racket/struct.rktl b/pkgs/racket-test-core/tests/racket/struct.rktl
index 891e560c8f..e7fbad629b 100644
--- a/pkgs/racket-test-core/tests/racket/struct.rktl
+++ b/pkgs/racket-test-core/tests/racket/struct.rktl
@@ -1106,6 +1106,12 @@
   (test #t procedure? acc)
   (test #t procedure? mut))
 
+;; ----------------------------------------
+;; Check that prefab struct size limit is enforced:
+
+(err/rt-test (make-prefab-struct '(foo 2999999999999999) 1))
+(err/rt-test (make-prefab-struct '(foo 5 bar 2999999999999999) 1))
+
 ;; ----------------------------------------
 
 (report-errs)
diff --git a/racket/src/racket/src/struct.c b/racket/src/racket/src/struct.c
index 9ea1fad9ab..596dcb7596 100644
--- a/racket/src/racket/src/struct.c
+++ b/racket/src/racket/src/struct.c
@@ -5364,7 +5364,7 @@ Scheme_Struct_Type *scheme_lookup_prefab_type(Scheme_Object *key, int field_coun
 {
   Scheme_Struct_Type *parent = NULL;
   Scheme_Object *a, *uninit_val, *mutables, *name;
-  int ucnt, icnt;
+  intptr_t ucnt, icnt;
   char *immutable_array = NULL;
 
   if (SCHEME_SYMBOLP(key))