improve open.pm layer class error message

diff --git a/lib/open.pm b/lib/open.pm
index a6b4250..74119ca 100644 (file)
--- a/lib/open.pm
+++ b/lib/open.pm
@@ -108,7 +108,7 @@ sub import {
            $in = $out = join(' ', @val);
        }
        else {
-           croak "Unknown PerlIO layer class '$type'";
+           croak "Unknown PerlIO layer class '$type' (need IN, OUT or IO)";
        }
     }
     ${^OPEN} = join("\0", $in, $out);