Crazy Syntax Error

Question

this is the error that i am getting:

FormatException was unhandled

Input string was not in a correct format.

i am getting this from this line of code:

double b = double.Parse(Counter.Text) - .01;

so i rewrote it as this:

string b = Convert.ToString((Convert.ToDouble(Counter.Text)) - .01);

and got the same error

i am using this code like this:

private void Timer()

        {

            //Starts a conditional loop

            //Conditions are that 'TimeInterval' is set to 1

            if (Convert.ToBoolean(TimeInterval = 1))

            {

                //Creates 'b' and sets it to what the current text in 'Counter'

                //and takes .01 from it

                string b = Convert.ToString((Convert.ToDouble(Counter.Text)) - .01);

                //Sets the text displayed in the Points to 'b'

                Counter.Text = Convert.ToString(b);

            }

        }

this is for a small game in my programming class and is the last bit of code i need

any help solving this will be apreciated