"Error converting data type nvarchar to numeric" even if my data types are correct

My code experienced a weird problem. Last night, I ran my program. Everything was fine. Today, I ran the program, and got an error

Error converting data type nvarchar to numeric

even if I didn't change any settings or configurations in my database nor my code.

Code:

private void btnAdd_Click(object sender, EventArgs e)
{
    SqlCommand cmd;
    SqlConnection con;

    con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Kim\Documents\SMEMCO.mdf;Integrated Security=True");
    con.Open();

    cmd = new SqlCommand("INSERT INTO LoanRecord (EmployeeName, EmployeeNumber, Department, LoanAmount, YearsToPay, MonthlyPayment, TotalPayment) VALUES (@EmployeeName, @EmployeeNumber, @Department, @LoanAmount, @YearsToPay, @MonthlyPayment, @TotalPayment)", con);

    cmd.Parameters.AddWithValue("@EmployeeName", txtName.Text);
    cmd.Parameters.AddWithValue("@EmployeeNumber", txtEmpno.Text);
    cmd.Parameters.AddWithValue("@Department", txtDept.Text);
    cmd.Parameters.AddWithValue("@LoanAmount", amount);
    cmd.Parameters.AddWithValue("@YearsToPay", txtYears.Text);
    cmd.Parameters.AddWithValue("@MonthlyPayment", monthly);
    cmd.Parameters.AddWithValue("@TotalPayment", total);

    cmd.ExecuteNonQuery();

    MessageBox.Show("Data successfully added to database.");
}

The exception was called in this form. I'm sure that my data types are appropriate.

I tried doing this but it threw an error

Input string was not in correct format

Code:

decimal amount = decimal.Parse(txtAmount.Text);
cmd.Parameters.AddWithValue("@Amount", amount);

Yesterday's code didn't have that code above and yet, it worked fine. What happened and what did I do wrong?

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