unserialize() expects parameter 1 to be string, array given

I created a pollfield with following options:
Maximum number of Response-fileds: 2
Status: active
Duration: 4 weeks
Anonymous voting: Not allowed
Cancel vote: allowed
Results: allowed, single choice
Two prefilled choices

When I was filling out the form for the first time, I chose Max-response fileds: 1, since I didn't know, what does it mean. But I already filled out the form with two choices and saved the form. Then I got a warning, that I need at least 2 Max-response fields. So I corrected it and saved the form.

Than I tried to add a new content, which would include the new created poll. Than I noticed, that one of those choice-fields was empty, so I filled it out with a proper option. And saved the new content.

And than I got the following warning:
warning: unserialize() expects parameter 1 to be string, array given in

/path/to/my/drupal/sites/all/modules/pollfield/pollfield.module on line 476.

So I changed the pollfield-options in the general content preferences and set this second choice, so I don't have to fill it out every time creating this new type of content.

Than I once more tried to add a new content of this type, but the warning showed up again.

One more thing: the poll works as it should and the votes are being counted. But since I'm not the enduser who will be creating the polls, I would be very happy, if this warning disappeared.