diff --git a/collects/mzlib/foreign.ss b/collects/mzlib/foreign.ss
index 0d320ed..8c7ccf7 100644
--- a/collects/mzlib/foreign.ss
+++ b/collects/mzlib/foreign.ss
@@ -1466,12 +1466,15 @@
                         list->TYPE list*->TYPE TYPE->list TYPE->list*)))))))
   (define (identifiers? stx)
     (andmap identifier? (syntax->list stx)))
-  (define (_-identifier? stx)
-    (and (identifier? stx)
-         (regexp-match #rx"^_.+" (symbol->string (syntax-e stx)))))
+  (define (_-identifier? id stx)
+    (and (identifier? id)
+         (or (regexp-match #rx"^_." (symbol->string (syntax-e id)))
+             (raise-syntax-error #f "cstruct name must begin with a `_'"
+                                 stx id))))
   (syntax-case stx ()
     [(_ _TYPE ([slot slot-type] ...))
-     (and (_-identifier? #'_TYPE) (identifiers? #'(slot ...)))
+     (and (_-identifier? #'_TYPE stx)
+          (identifiers? #'(slot ...)))
      (make-syntax #'_TYPE #f #'(slot ...) #'(slot-type ...))]
     [(_ (_TYPE _SUPER) ([slot slot-type] ...))
      (and (_-identifier? #'_TYPE) (identifiers? #'(slot ...)))