diff --git a/collects/parser-tools/yacc.ss b/collects/parser-tools/yacc.ss
index 56f0283..a54223c 100644
--- a/collects/parser-tools/yacc.ss
+++ b/collects/parser-tools/yacc.ss
@@ -282,8 +282,8 @@
                                    token-index)
                       (begin
                         (if src-pos
-                            (err #t (token-name tok) (token-value tok) (cadr ip) (caddr ip))
-                            (err #t (token-name tok) (token-value tok)))
+                            (err #f (token-name tok) (token-value tok) (cadr ip) (caddr ip))
+                            (err #f (token-name tok) (token-value tok)))
                         (raise-read-error (format "parser: got token of unknown type ~a" (token-name tok))
                                           #f #f #f #f #f)))))))
       (lambda (get-token)