NaN MASM Error?

[hoofhamples]

So i receive this NaN error (notanumber) when i try to add 2 values of which are floats. How would i fix this error?

i have this as my info...

leftvalue      REAL8   ?           ;these ? values are actual large numbers so i assume overflow?  
leftmidvalue   REAL8   ?


calculate_left:
    fld leftvalue
    fld leftmidvalue
    fadd
    fst leftside
    fld leftside
    call WriteFloat