Cos coming up with NaN

[dantm]

 BeforeInverse= ((2nd.Text ^2 + 3rd.Text ^ 2 - 1st.Text ^ 2) / (2 * 2nd.Text * 3rd.Text))
                Calculation.Text = Math.Acos(BeforeInverse/ Math.PI * 180) 

The above is using the cosine formula which has been rearranged to solve for the angle.
When i calculate, it comes up with NaN and i'm not dividing by 0. What does NaN mean and is there an error with the above code?