Float.Parse question

I'm trying to convert a string, entered by the player into a Gui.textfield, into a float. However, Im constantly getting this error: "FormatException: Invalid format. System.Double.Parse (System.String s, NumberStyles style, IFormatProvider provider) System.Single.Parse (System.String s)"

My code is as follows:

string str_inputAmount = "";
                        str_inputAmount = GUI.TextField(new Rect(v3_screenPos.x + 55f, v3_screenPos.y - (i * 55f), 50f, 50f), str_inputAmount, 4);
                        float inputAmount = 0f;
                        inputAmount = float.Parse(str_inputAmount);

What could my solution be?

http://answers.unity3d.com/questions/47763/convert-string-to-float-or-double-errors

              string myString = "15.5";
              float number = (float) int.Parse(myString);

              float num = System.Convert.ToSingle( myString );//or this