diff --git a/src/pkg/net/iprawsock.go b/src/pkg/net/iprawsock.go
index 92c299e4fc..241be15095 100644
--- a/src/pkg/net/iprawsock.go
+++ b/src/pkg/net/iprawsock.go
@@ -288,7 +288,7 @@ func readProtocols() {
 func netProtoSplit(netProto string) (net string, proto int, err os.Error) {
 	onceReadProtocols.Do(readProtocols)
 	i := last(netProto, ':')
-	if i+1 >= len(netProto) { // no colon
+	if i < 0 { // no colon
 		return "", 0, os.ErrorString("no IP protocol specified")
 	}
 	net = netProto[0:i]