diff --git a/pkgs/racket-test-core/tests/racket/will.rktl b/pkgs/racket-test-core/tests/racket/will.rktl
index 801acba0e7..1250b290ac 100644
--- a/pkgs/racket-test-core/tests/racket/will.rktl
+++ b/pkgs/racket-test-core/tests/racket/will.rktl
@@ -247,6 +247,10 @@
 (unless (eq? 'cgc (system-type 'gc))
   (define s (make-semaphore))
   (define c (make-custodian))
+  (define bits (let loop ([bits 29])
+                 (if (fixnum? (expt 2 bits))
+                     bits
+                     (loop (sub1 bits)))))
   (define t (parameterize ([current-custodian c])
               (thread (lambda ()
                         (semaphore-wait s)
diff --git a/racket/src/cs/rumble/memory.ss b/racket/src/cs/rumble/memory.ss
index 1bf82626fd..19b43a1c8d 100644
--- a/racket/src/cs/rumble/memory.ss
+++ b/racket/src/cs/rumble/memory.ss
@@ -123,7 +123,7 @@
           ;; `post-allocated+overhead` seems to be too long a wait, because
           ;; that value may include underused pages that have locked objects.
           ;; Using just `post-allocated` is too small, because it may force an
-          ;; immediate major GC too soon. Split the difference.
+            ;; immediate major GC too soon. Split the difference.
           (set! trigger-major-gc-allocated (* GC-TRIGGER-FACTOR (- post-allocated (bytes-finalized))))
           (set! trigger-major-gc-allocated+overhead (* GC-TRIGGER-FACTOR post-allocated+overhead)))
         (update-eq-hash-code-table-size!)
@@ -490,6 +490,7 @@
 
 (define/who (make-phantom-bytes k)
   (check who exact-nonnegative-integer? k)
+  (guard-large-allocation who "byte string" k 1)
   (let ([ph (create-phantom-bytes (make-phantom-bytevector k))])
     (when (or (>= (bytes-allocated) trigger-major-gc-allocated)
               (>= (current-memory-bytes) trigger-major-gc-allocated+overhead))
@@ -499,6 +500,8 @@
 (define/who (set-phantom-bytes! phantom-bstr k)
   (check who phantom-bytes? phantom-bstr)
   (check who exact-nonnegative-integer? k)
+  (when (> k (phantom-bytevector-length (phantom-bytes-pbv phantom-bstr)))
+    (guard-large-allocation who "byte string" k 1))
   (set-phantom-bytevector-length! (phantom-bytes-pbv phantom-bstr) k))
 
 ;; ----------------------------------------