Create a Standard-Plausabilitycheck, switch to the source code editor and check the relevant variables like this:
The check pops up, if something has been put into the text field (v_2) but nothing has been selected in the scale and vice versa. Please make sure that only pure Javascript-checks are used, the PHP Check would cause problems within this constellation.
if ( ( v_1 < 1 AND v_2 != '') OR ( v_1 > 0 AND v_2 == ''))
print "An error has occured!";