diff --git a/collects/macro-debugger/syntax-browser/widget.rkt b/collects/macro-debugger/syntax-browser/widget.rkt
index fd75cdd..b84fddc 100644
--- a/collects/macro-debugger/syntax-browser/widget.rkt
+++ b/collects/macro-debugger/syntax-browser/widget.rkt
@@ -117,8 +117,8 @@
             (send -text change-style clickback-style a b)))))
 
     (define/public (add-syntax stx
-                               #:binders [binders #f]
-                               #:shift-table [shift-table #f]
+                               #:binders [binders '#hash()]
+                               #:shift-table [shift-table '#hash()]
                                #:definites [definites #f]
                                #:hi-colors [hi-colors null]
                                #:hi-stxss [hi-stxss null]
diff --git a/collects/macro-debugger/view/step-display.rkt b/collects/macro-debugger/view/step-display.rkt
index 51259a6..d2f6f7d 100644
--- a/collects/macro-debugger/view/step-display.rkt
+++ b/collects/macro-debugger/view/step-display.rkt
@@ -84,9 +84,9 @@
              (show-poststep step shift-table)]))
 
     (define/public (add-syntax stx
-                               #:binders [binders #f]
+                               #:binders [binders '#hash()]
                                #:definites [definites #f]
-                               #:shift-table [shift-table #f])
+                               #:shift-table [shift-table '#hash()])
       (send/i sbview sb:syntax-browser<%> add-syntax stx
               #:binders binders
               #:definites definites