diff --git a/match.c b/match.c
index 7aebe2fe..c8de25c9 100644
--- a/match.c
+++ b/match.c
@@ -5,6 +5,8 @@
 #include <stdarg.h>
 #include <string.h>
 
+#include <stdio.h>
+
 #include "lexer.h"
 
 static void
@@ -25,6 +27,7 @@ match_func_init(struct match_func *mf)
 		}
 	}
 
+	fprintf(stderr, "XXX %s: no callback for '%s'\n", __func__, mf->expr);
 	__builtin_trap(); /* UNREACHABLE */
 }