--- typemap
+++ typemap
@@ -37,9 +37,9 @@ T_LOOP
 
 T_WATCHER
 	if (!(SvROK ($arg) && SvOBJECT (SvRV ($arg))
-              && (SvSTASH (SvRV ($arg)) == stash_" . ($type =~ /ev_(\S+)/, "$1") . "
-                  || sv_derived_from ($arg, \"EV::" . ($type =~ /ev_(\S+)/, ucfirst "$1") . "\"))))
-          croak (\"object is not of type EV::" . ($type =~ /ev_(\S+)/, ucfirst "$1") . "\");
+              && (SvSTASH (SvRV ($arg)) == stash_${ ($type =~ /ev_(\S+)/, \"$1") }
+                  || sv_derived_from ($arg, \"EV::${ ($type =~ /ev_(\S+)/, \ucfirst "$1") }\"))))
+          croak (\"object is not of type EV::${ ($type =~ /ev_(\S+)/, \ucfirst "$1") }\");
 	$var = ($type)SvPVX (SvRV ($arg));
 
 OUTPUT