diff --git a/chameleonic.p8 b/chameleonic.p8
index 95b65e8..30b30c6 100644
--- a/chameleonic.p8
+++ b/chameleonic.p8
@@ -62,7 +62,7 @@ function fmt(f, ...)
     elseif m == "!" then
      out ..= tostring(p)
     else
-     out ..= "(?!:"..m..":"..tostring(p)..")"
+     assert(false, tostr(m).."is not a formatting directive")
     end
    end
    out ..=sub(s,2)