diff --git a/collects/errortrace/stacktrace.ss b/collects/errortrace/stacktrace.ss
index 017b37d76f..c4fb5478ef 100644
--- a/collects/errortrace/stacktrace.ss
+++ b/collects/errortrace/stacktrace.ss
@@ -516,7 +516,7 @@
                 ;; It's a null:
                 expr]
                [(syntax-case* expr (#%plain-app void)
-                              (if phase
+                              (if (positive? phase)
                                 free-transformer-identifier=?
                                 free-identifier=?)
                   [(#%plain-app void) #t]