Undefined index error notice when VRF is set to required

I use Views Reference Filter as an exposed filter to restrain values for a Jump menu. Jump menu items are conditioned by the values selected in the exposed VRF.

The construction works as expected if the reference filter is not required. However, when the reference filter is set to required, I receive an undefined index notice.

Notice: Undefined index: c in views_handler_filter->store_exposed_input() (line 1395 of /.... /sites/all/modules/views/handlers/views_handler_filter.inc).

The strange thing is that despite the error notice, the view actually works, that is, the items are listed according to first autoselected value of the VRF. I am not sure whether this is a problem of this module or a Views problem.

Thanks for any suggestion.