How to solve this? Input string was not in a correct format.in ASP.NET

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C#

ASP.NET

I have two textBoxs and one button, textbox1 is ascept Number and textbox2 is convert them in Words. when i click button1 as a clear the field it shows the error-

Input string was not in a correct format

error dialogbox shows here error-

string word = ConvertNumbertoWords(Convert.ToInt32(txtamount.Text));

What I have tried:

private void buttonClear_Click(object sender, EventArgs e)
{

textBox1.Text = String.Empty;
txtsrno.Text = String.Empty;
txtdescription.Text = String.Empty;
txtquantity.Text = String.Empty;
txtrate.Text = String.Empty;
comboBox1.Text = String.Empty;
txtamount.Text = String.Empty;
txtword.Text = String.Empty;

string word = ConvertNumbertoWords(Convert.ToInt32(txtamount.Text));
txtword.Text = word;

}

string word = ConvertNumbertoWords(Convert.ToInt32(txtamount.Text));