diff --git a/collects/mred/private/messagebox.rkt b/collects/mred/private/messagebox.rkt
index 38733d0ffb..e0de7fcabf 100644
--- a/collects/mred/private/messagebox.rkt
+++ b/collects/mred/private/messagebox.rkt
@@ -46,15 +46,19 @@
                    who title message
                    button1 button2 button3
                    parent style close-result
-                   check? two-results? check-message))])
-        (if (eq? (current-thread) (wx:eventspace-handler-thread (wx:current-eventspace)))
+                   check? two-results? check-message))]
+	    [es (if parent
+		    (send parent get-eventspace)
+		    (wx:current-eventspace))])
+        (if (eq? (current-thread) (wx:eventspace-handler-thread es))
             ;; In the right thread:
             (go)
             ;; Not in the right thread:
             (let ([ch (make-channel)])
-              (wx:queue-callback
-               (lambda ()
-                 (channel-put ch (call-with-values go list))))
+	      (parameterize ([wx:current-eventspace es])
+                (wx:queue-callback
+                 (lambda ()
+                   (channel-put ch (call-with-values go list)))))
               (apply values (channel-get ch)))))))
 
   (define create-message-box/custom
diff --git a/collects/scribblings/gui/dialog-funcs.scrbl b/collects/scribblings/gui/dialog-funcs.scrbl
index 794e533449..d6300abc07 100644
--- a/collects/scribblings/gui/dialog-funcs.scrbl
+++ b/collects/scribblings/gui/dialog-funcs.scrbl
@@ -221,7 +221,8 @@ The class that implements the dialog provides a @scheme[get-message]
  @scheme[get-top-level-windows] function.)
 
 The @racket[message-box] function can be called int a thread other
- than the current eventspace's handler thread, in which case the
+ than the handler thread of the relevant eventspace (i.e., the eventspace of
+ @racket[parent], or the current eventspace if @racket[parent] is @racket[#f]), in which case the
  current thread blocks while the dialog runs on the handler thread.}
 
 @defproc[(message-box/custom [title label-string?]
@@ -305,7 +306,8 @@ The class that implements the dialog provides a @scheme[get-message]
 @scheme[get-top-level-windows] function.)
 
 The @racket[message-box/custom] function can be called int a thread
- other than the current eventspace's handler thread, in which case the
+ other than the handler thread of the relevant eventspace (i.e., the eventspace of
+ @racket[parent], or the current eventspace if @racket[parent] is @racket[#f]), in which case the
  current thread blocks while the dialog runs on the handler thread.}
 
 @defproc[(message+check-box [title label-string?]