diff --git a/pkgs/racket-test-core/tests/racket/struct.rktl b/pkgs/racket-test-core/tests/racket/struct.rktl
index e7fbad629b..5d259ce48f 100644
--- a/pkgs/racket-test-core/tests/racket/struct.rktl
+++ b/pkgs/racket-test-core/tests/racket/struct.rktl
@@ -1112,6 +1112,13 @@
 (err/rt-test (make-prefab-struct '(foo 2999999999999999) 1))
 (err/rt-test (make-prefab-struct '(foo 5 bar 2999999999999999) 1))
 
+;; ----------------------------------------
+;; Check that prefab mutable-field spec makes sense for size:
+
+(err/rt-test (make-prefab-struct '(foo 5 (1 #f) #(1) bar 2 #(99999)) 1 2 3 4 5 6 7 8))
+(test #t struct? (make-prefab-struct '(foo 5 (1 #f) #(1) bar 2 #()) 1 2 3 4 5 6 7 8))
+(test #t struct? (make-prefab-struct '(foo 5 (1 #f) #(1) bar 0 #()) 1 2 3 4 5 6))
+
 ;; ----------------------------------------
 
 (report-errs)
diff --git a/racket/src/racket/src/struct.c b/racket/src/racket/src/struct.c
index 596dcb7596..88808e2258 100644
--- a/racket/src/racket/src/struct.c
+++ b/racket/src/racket/src/struct.c
@@ -5446,7 +5446,11 @@ Scheme_Struct_Type *scheme_lookup_prefab_type(Scheme_Object *key, int field_coun
       return NULL;
     name = a;
 
-    immutable_array = mutability_data_to_immutability_data(icnt + ucnt, mutables);
+    if ((icnt + ucnt) || (mutables && SCHEME_VEC_SIZE(mutables))) {
+      immutable_array = mutability_data_to_immutability_data(icnt + ucnt, mutables);
+      if (!immutable_array)
+        return NULL;
+    }
 
     if (parent && (icnt + parent->num_slots > MAX_STRUCT_FIELD_COUNT))
       return NULL;