Checkbox not workink

Hi,

In my website, when adding a Meta Field, choosing radio or dropdown works fine. However, if I just change to checkbox, no result is found. I noticed that there is a warning:

stripslashes() expects parameter 1 to be string, array given
at
wp-content/plugins/ultimate-wp-query-search-filter/classes/uwpqsf-process-class.php:81

Looks like there is something wrong on building the array.
The URL when radio is set is:

https://beiras.localhost/?unonce=d70ab3da54&uformid=820&s=uwpsfsearchtrg&cmf%5B0%5D%5Bmetakey%5D=duracao&cmf%5B0%5D%5Bcompare%5D=11&cmf%5B1%5D%5Bmetakey%5D=tipo&cmf%5B1%5D%5Bcompare%5D=1&cmf%5B1%5D%5Bvalue%5D=Fotografia&skeyword=

and 1 result is found. When checkbox is set, with the same single option as above, the URL is:

https://beiras.localhost/?unonce=d70ab3da54&uformid=820&s=uwpsfsearchtrg&cmf%5B0%5D%5Bmetakey%5D=duracao&cmf%5B0%5D%5Bcompare%5D=11&cmf%5B1%5D%5Bmetakey%5D=tipo&cmf%5B1%5D%5Bcompare%5D=1&cmf%5B1%5D%5Bvalue%5D%5B%5D=Fotografia&skeyword=

and nothing is found. The only difference is a “[]” after “value” in the second case.

thanks in advance,
alan