diff --git a/handshake_client.go b/handshake_client.go
index f8db662..38d01bf 100644
--- a/handshake_client.go
+++ b/handshake_client.go
@@ -848,7 +848,7 @@ func hostnameInSNI(name string) string {
 	if len(host) > 0 && host[0] == '[' && host[len(host)-1] == ']' {
 		host = host[1 : len(host)-1]
 	}
-	if i := strings.LastIndex(host, "%"); i > 0 {
+	if i := strings.LastIndexByte(host, '%'); i > 0 {
 		host = host[:i]
 	}
 	if net.ParseIP(host) != nil {
diff --git a/tls.go b/tls.go
index 615d1e5..1c91dae 100644
--- a/tls.go
+++ b/tls.go
@@ -122,7 +122,7 @@ func DialWithDialer(dialer *net.Dialer, network, addr string, config *Config) (*
 		return nil, err
 	}
 
-	colonPos := strings.LastIndex(addr, ":")
+	colonPos := strings.LastIndexByte(addr, ':')
 	if colonPos == -1 {
 		colonPos = len(addr)
 	}