Error in Evaluating IIP2 with Monte Carlo

Hello All,

I would like to know why Monte carlo iteration is being terminated prematurely while computing IIP2 due to the following error:

ERROR (SPECTRE-8003): mc1_subckts: Error evaluating ocean expression `MONTE_EXPR_1=ipnVRI((v("/Voutp" ?result "hbac") - 
            v("/Voutn" ?result "hbac")) '-1 '0 ?rport 50.0 ?ordspur 2)'.

I am presently using cadence Virtuoso IC6.1.3.500.13

for the spectre :  sub-version 7.1.1.187.isr11

Thanks as I look forward to possible suggestions and solutions.