REALLOC error

All,
 
I'm having a problem with using REALLOC (and its equivalent %REALLOC):
 
It's exactley the same as David Mosrris mentioned in this thread:
http://archive.midrange.com/rpg400-l/199802/msg00001.html
 
Essentially, I'm processing a binary tree where every tree item has the
following structure:

D Item            DS                  Based(Item@)
D  From@                          *               
D  Left@                          *               
D  Right@                         *               
D  Data@                          *               

and where I also have the following field:

D ItemData        S          32757A   Varying Based(ItemData@)

As I add elements to the tree (elements are Varying data strings up to
1000 bytes long, with the first 20 characters being a key), I check if
an item already exists with the same key. If not, I add a new item,
otherwise I overwrite the existing item's data with the new data.

The processing to add/update data is as follows (where InString is the
new data string):

C                   Eval      VLen = %len( InString ) + 2        
 *                                                                 
 * Store item data                                  
 *                                                                 
C                   If        Data@ <> *Null                 
C                   Eval      Data@ = %realloc( Data@ : VLen )
C                   Else                                           
C                   Eval      Data@ = %alloc( VLen )         
C                   Endif                                          
 *                                                                 
C                   If        Data@ = *Null                  
C                   Exsr      *PSSR                                
C                   Endif                                          
 *                                                                 
C                   Eval      ItemData@ = Data@              
C                   Eval      ItemData  = String

Now all the %alloc's work fine, as does the first %realloc. However,
subsequent
%realloc's fail with a CEE0810 ("The starting address for reallocation
is not valid."). However, I've done all the %alloc's/%realloc's...

What am I missing? I'm on a V5R2 box, compiling back to V5R1, but this
also seems to fail at V5R2.

TIA,

Rory