diff --git a/private/pure-function.rkt b/private/pure-function.rkt
index baee07d..1bc3f83 100644
--- a/private/pure-function.rkt
+++ b/private/pure-function.rkt
@@ -8,6 +8,7 @@
          (only-in typed/racket/unsafe unsafe-require/typed)
          (prefix-in te: type-expander)
          phc-toolkit
+         version-case
          (for-syntax (rename-in racket/base [... …])
                      racket/match
                      syntax/modcollapse
@@ -21,6 +22,12 @@
                      phc-toolkit/untyped
                      "fully-expanded-grammar-no-set.rkt"))
 
+(version-case
+ [(version< (version) "6.90.0.29")
+  (begin)]
+ [else
+  (require racket/unsafe/undefined)])
+
 (unsafe-require/typed
  "pure-unsafe.rkt"
  [promise/pure/maybe-stateful? (→ Any Boolean : Promise)]
@@ -221,6 +228,15 @@
     ;; To allow pure functions which return pure functions, we need to allow
     ;; check-immutable/c itself
     [(eq? x check-immutable/error)               #t]
+    ;; racket/unsafe/undefined is used in the expanded code for functions with
+    ;; opetional arguments. We allow it here (for now), even though it is
+    ;; unsafe, because the user (or a library) would need to explicitly require
+    ;; it to cause problems. Otherwise, it will only appear in code generated by
+    ;; typed/racket.
+    [(version-case
+       [(version< (version) "6.90.0.29") #f]
+       [else                             (eq? x unsafe-undefined)])
+     #t]
     ;; Otherwise, fail early before mutation causes problems
     [else (begin (other) #f)]))
 
diff --git a/test/test-external-mutation.rkt b/test/test-external-mutation.rkt
index 8207f9f..e4ea3fc 100644
--- a/test/test-external-mutation.rkt
+++ b/test/test-external-mutation.rkt
@@ -4,7 +4,6 @@
 
 ;; This file checks that externally mutating a free variable on which a pure
 ;; function or promise depends does not affect the function's result.
-
 (check-equal? (let ([x 1])
                 (define d (delay/pure/stateful (add1 x)))
                 (list (begin (set! x -10) (force d))
@@ -78,4 +77,11 @@
                 (define-pure/stateful (d #:kw [opt : Number x]) (add1 opt))
                 (list (begin (set! x -10) (d))
                       (begin (set! x -11) (d))))
-              '(2 2))
\ No newline at end of file
+              '(2 2))
+
+;; Check that this doesn't cause a run-time error due to the internal use of
+;; unsafe-undefined in the expanded function with optional arguments (starting
+;; from Racket 6.90.0.29)
+(define z 1)
+(define-pure/stateless (d [opt : Number z]) (void))
+(d)
\ No newline at end of file