diff --git a/tds.go b/tds.go
index 832c4fd2..67139c6a 100644
--- a/tds.go
+++ b/tds.go
@@ -838,6 +838,12 @@ func connect(ctx context.Context, c *Connector, log optionalLogger, p connectPar
 		defer cancel()
 	}
 	// if instance is specified use instance resolution service
+	if p.instance != "" && p.port != 0 {
+		// both instance name and port specified
+		// when port is specified instance name is not used
+		// you should not provide instance name when you provide port
+		log.Println("WARN: You specified both instance name and port in the connection string, port will be used and instance name will be ignored");
+	}
 	if p.instance != "" && p.port == 0 {
 		p.instance = strings.ToUpper(p.instance)
 		d := c.getDialer(&p)
diff --git a/tds_test.go b/tds_test.go
index e725a668..0e39e224 100644
--- a/tds_test.go
+++ b/tds_test.go
@@ -227,12 +227,18 @@ func (l testLogger) Println(v ...interface{}) {
 func TestConnect(t *testing.T) {
 	checkConnStr(t)
 	SetLogger(testLogger{t})
-	conn, err := sql.Open("mssql", makeConnStr(t).String())
+	conn, err := sql.Open("mssql", os.Getenv("SQLSERVER_DSN"))
 	if err != nil {
 		t.Error("Open connection failed:", err.Error())
 		return
 	}
 	defer conn.Close()
+	row := conn.QueryRow("select 1")
+	var val int
+	err = row.Scan(&val)
+	if err != nil {
+		t.Error("Scan failed:", err.Error())
+	}
 }
 
 func simpleQuery(conn *sql.DB, t *testing.T) (stmt *sql.Stmt) {