add nil check and test

client/client.go

@@ -685,6 +685,9 @@ func New() *Client {
 
 // NewWithClient creates and returns a new Client object from an existing client.
 func NewWithClient(c *fasthttp.Client) *Client {
+	if c == nil {
+		panic("fasthttp.Client must not be nil")
+	}
 	return &Client{
 		fasthttp: c,
 		header: &Header{

client/client_test.go

@@ -58,12 +58,24 @@ func startTestServerWithPort(t *testing.T, beforeStarting func(app *fiber.App))
 func Test_New_With_Client(t *testing.T) {
 	t.Parallel()
 
-	c := &fasthttp.Client{
-		MaxConnsPerHost: 5,
-	}
-	client := NewWithClient(c)
+	t.Run("with valid client", func(t *testing.T) {
+		t.Parallel()
+
+		c := &fasthttp.Client{
+			MaxConnsPerHost: 5,
+		}
+		client := NewWithClient(c)
+
+		require.NotNil(t, client)
+	})
 
-	require.NotNil(t, client)
+	t.Run("with nil client", func(t *testing.T) {
+		t.Parallel()
+
+		require.PanicsWithValue(t, "fasthttp.Client must not be nil", func() {
+			NewWithClient(nil)
+		})
+	})
 }
 
 func Test_Client_Add_Hook(t *testing.T) {