Well, if you used the example as you typed it, then you have a typo;
there's an extra bracket at the end: [MTIC_PROD_VEND[ 1]]] should be
[MTIC_PROD_VEND[ 1]].
Of course, if it still doesn;t work, you might try using standard
quotation marks, e.g.:
"MTIC_PROD_VEND[ 1]"