The integrator block allows to integrate an input signal. Before using it set its initial state with

  setInitCondition(T val);

An integrator can be enabled or disabled.

  enable();
  disable();

This, for instance, helps to stop integrating up a given input while handling an exception. Please note that an integrator is in the disabled state upon creation.
Without setting the initial state to a meaningful value, the output will be nan even after enabling the block.