Error on using Modelica in Acttivate

Dear Experts,

When I ran a Modelica script through MoCustomBlock in Activate, Activate issued the following error:

[fmi2GetEventIndicators]: Error at t=7.5262725398123784e+000: index-1 and derivative evaluation failure

Failed in updating zero-crossing surfaces.

In what case, is this error issued and what should I fix generally? 

Regards,

Minoru Yubuchi