diff --git a/IkiWiki.pm b/IkiWiki.pm
index 2815a5c4c..051c11b43 100644
--- a/IkiWiki.pm
+++ b/IkiWiki.pm
@@ -1176,7 +1176,8 @@ sub preprocess ($$$;$$) { #{{{
 		}sx;
 	}
 
-	$content =~ s{$regex}{$handle->($1, $2, $3, $4)}eg;
+	# $4 can be undef if the directive was [[!foo]]
+	$content =~ s{$regex}{$handle->($1, $2, $3, ($4 or ""))}eg;
 	return $content;
 } #}}}