Varying length problem on Procedure

I getting the following error: Length of varying length variable is out
of range
I receive this error on the following statement :
C                   EVAL      #First = GLowToUp(%SUBST(UMFNM:1:1))

     P gLowToUp        B                   EXPORT
     D                 PI           100A   Varying
     D  FieldIn                     100A   Value
     D                                     Varying
      *
     D LCase           C
Const('abcdefghijklmnopqrstuvwxyz')
     D UCase           C
Const('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
     D RESULT          s            100A   Varying
      *
     C                   Eval      %len(RESULT) = %len(FieldIn)
     C     LCase:UCase   XLATE     FieldIn       RESULT
     C                   Return    RESULT
     P GLowToUp        E                                            

The program normally runs in Batch, but if I run the program
interactively I don't get this error, Maybe its just coincidence.  
Can someone tell me whats wrong?

Thanks
Michael Smith
iSeries.mySeries.