Assignment to varray element fail inexplicably

Hi,

I have PL/SQL script containing the following sequence of commands:

....

node_seqX.DELETE;
i:=cntX.FIRST;
node_seqX.EXTEND;
node_seqX(1):=i;

....

The definitions of the collections are as follows:

TYPE node_seqT IS VARRAY(300) OF NUMBER;

TYPE cnt_typeT IS TABLE OF NUMBER
  INDEX BY PLS_INTEGER;

cntX cnt_typeT;

node_seqX node_seqT:=node_seqT();

The script ran many times without problems, but now the last statement causes the script to hang.

The value of cntX.FIRST is 480620, if I put this value in the last statement instead of i, it runs without problems.

I cannot make out what the cause could be.