Pointer arithmetic

Listees,

Does anyone know if the following should work?  Based on the manual I think it 
should work I faintly remember doing this in the past, but I can't get to work 
now.

D OffSet                        10I 0
D Ptr1                            *
D Ptr2                            *
...
C                   EVAL      OffSet = Ptr1 - Ptr2

I get the following message:

*RNF7421 30      1 Operands are not compatible with the type of operator.
                     Cause . . . . . :   Operands must be of class character or
                       graphic for character operations, numeric for arithmetic
                       operations, and indicator for logical and relational
                       operations. In a comparison operation, both operands must
                       have the same data type.  The specification is ignored.
                     Recovery  . . . :   Specify the correct operand classes and
                       data types. Compile again.

The manual states the following:

  Operator          Operand 1 Type     Operand 2 Type    Result Type      
  - (subtract       Basing Pointer     Basing Pointer    Numeric          
  pointers)                                                               

What am I missing here?  I have coded a work-around but I want to understand 
why this doesn't work.

Thanks,

David Morris
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                                
                                                                     
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