Small Installment Calculator Assignment - Getting A NaN Result

I have a small installment calculator assignment, I have only used VB6 before but i am required to use .NET (using Visual Basic 2008 Express).

There are four fields:

1. Loan Amount (10,000.00 - 250,000.00)

2. Loan Term (5 -25 years)

3. Loan Interest Rate ( Input to 2 decimial places)

4. Payment Frequency (12, 4, 2, 1)

I need to calculate an Effective Rate as:

CODE;

Then calculate an Installment Amount:

CODE:

Where:

CODE:

And finally display installment amount on the form.

This is the code I have so far, but I am getting a NaN result!

CODE: