Getting Nan

Hi,

I'm getting NaN as answer for expressions like below

MN1=WTAP/( SQRT(GC*GAMA1/R1)*(1.0+(GAMA1-1.0)/2.0*
     &    MN1**2.0)**((GAMA1+1.0)/(2.0*(1.0-GAMA1))) )

All are real variables and they have real values.

When I simplify the expression , it gives the correct answer. But then I am getting Nan as answer for a another expression. Then I have to simplify it also. 

Please help me.

Thank you. 

Intel visual Fortran Compiler