MATAGPAT strange behavior?

When I get the program activation list using MATAGPAT on V5R1, and the size
provided (pos 1-4) is less than the size available (pos 5-8), then MATAGPAT
destroys my operand 1 space pointer to the receiver template.

dcl dd     Receiver   auto char(64) bdry(16);
dcl dd     *          def(Receiver) pos(1) bin(64);
dcl spcptr ?Receiver  auto init(Receiver);

matagpat ?Receiver, GroupMark, X"02";

/* ?Receiver is now null */




dcl dd     Receiver   auto char(8192) bdry(16);
dcl dd     *          def(Receiver) pos(1) bin(8192)
dcl spcptr ?Receiver  auto init(Receiver);

matagpat ?Receiver, GroupMark, X"02";

/* ?Receiver remains okay */