So I want to write a code that if the textbox (txtPayment.Text) is entered but the textbox is empty an error message should pop up [duplicate]

This question already has an answer here:

• C# Why won't this substring work? Error: Input string was not in a correct format 5 answers

This is the code for Key Press Enter

private void txtPayment_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (txtPayment.Text == null)
            {
                MessageBox.Show("Cash payment is empty", "Error Message!", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

            else if (e.KeyChar == (char)Keys.Enter)
            {
                payment();
            }
        }

and here's the code for payment method

private void payment()
        {
            Int32 val1 = Convert.ToInt32(txtTotal.Text);
            Int32 val2 = Convert.ToInt32(txtPayment.Text);
            Int32 val3 = val2 - val1;

            if (val2 < val1)
            {
                MessageBox.Show("Insufecient cash payment", "Error Message!", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtPayment.Text = "";
            }

            else
            {
                txtChange.Text = val3.ToString();
            }
        }

but I always get the error message "Input string was not in a correct format."