diff --git a/collects/racket/file.rkt b/collects/racket/file.rkt
index 7851d0c361..4420d9a2e0 100644
--- a/collects/racket/file.rkt
+++ b/collects/racket/file.rkt
@@ -185,9 +185,9 @@
     (raise-type-error 'call-with-file-lock/timeout "'shared or 'exclusive" kind))
   (unless (and (procedure? thunk) (= (procedure-arity thunk) 0))
     (raise-type-error 'call-with-file-lock/timeout "procedure (arity 0)" thunk))
-  (unless (and (procedure? thunk) (= (procedure-arity thunk) 0))
+  (unless (and (procedure? thunk) (= (procedure-arity failure-thunk) 0))
     (raise-type-error 'call-with-file-lock/timeout "procedure (arity 0)" failure-thunk))
-  (unless (path-string? lock-file)
+  (unless (or (not lock-file) (path-string? lock-file))
     (raise-type-error 'call-with-file-lock/timeout "path-string? or #f" lock-file))
   (unless (and (real? delay) (not (negative? delay)))
     (raise-type-error 'call-with-file-lock/timeout "non-negative real" delay))