Need a sanity check, please

["Robert Clay" <rclay@xxxxxxxxxxxx>]

We are at V5R1 so I have to jump through hoops to get things to work.  
 
Here's the situation:
 
We have MONTHID  (A  6) which represents YYYYMM, where YYYY is the year and MM 
is the month number.  Sample data would be '200404'.
 
I need to do some manipulation where this value is the key and I need to know 
the same month of the previous year.  So, I came up with this EVAL statement 
(in free format):
 
 
PASTMONTH = %char(atoi(%subst(MONTHID:1:4))-1) + %subst(MONTHID:5:2)
 
 
PASTMONTH is also (A  6).
 
 
I have the "atoi" C function prototyped:
 
 
h BNDDIR('QC2LE') 
 
 
d atoi            PR            10I 0 ExtProc('atoi')       
d  charValue                      *   VALUE Options(*STRING)
 
 
When I attempt to compile the program, I get this message on the above EVAL 
statement:
 
RNF0955  Item is not valid as the left-hand side of an EVAL operation.
 
Cause . . . . . :   The left-hand side of an EVAL operation must be an item   
  that can be modified.  This means that the following cannot be on the       
  left-hand side since they cannot be modified: figurative constants;         
  literals; named constants; lookahead fields; entry parameters with the CONST
  keyword specified; prototype names; certain special words and built-in      
  functions. The specification is ignored.                                    
Recovery  . . . :   Specify a variable in the left-hand side of the EVAL      
  operation or remove the specification. Compile again.                       
 
 
PASTMONTH is not a figurative constant, literal, named constant, lookahead 
field, etc.
 
It is a field in the file I am using:
 
fLUMTM     UF A E           K DISK    Rename( RLUMTM : LUMTMR ) USROPN
 
And, yes, I have the file opened on a prior statement due to the USROPN keyword.
 
Why the error?
 
It's probably something simple but I just can't see it.
 
Any help is greatly appreciated.
 
 
Robert