diff --git a/collects/deinprogramm/signature/signature.rkt b/collects/deinprogramm/signature/signature.rkt
index 425ff2dc51..6b851db4a6 100644
--- a/collects/deinprogramm/signature/signature.rkt
+++ b/collects/deinprogramm/signature/signature.rkt
@@ -373,7 +373,8 @@
    #:arbitrary-promise
    (delay (apply arbitrary-one-of =? cases))))
 
-(define-struct procedure-to-blame (proc syntax))
+(define-struct procedure-to-blame (proc syntax)
+  #:property prop:procedure 0)
 
 (define signature-key (gensym 'signature-key))