Bug in WWW::Mechanize or is it my fault?

Hello,

$url is a URI object

$mech->get($url);

if($mech->success())
{
$mech->submit_form
(
form_name => "mainForm",
fields => {
org => "D. melanogaster",
db => "dm2", <---------
this value is declared as illegal by Mechanize line 1169
type => "BLAT's guess",
sort => "query,score",
output => "hyperlink",
userSeq => "$alignmentQuery",
},
button => "Lucky" #"Submit"
);

The following way everything works well. LWP style:

$mech->post($url, {org => "D. melanogaster", db => "dm2", type =>
"BLAT's guess", sort => "query,score", output => "hyperlink", userSeq =>
"$alignmentQuery", Submit => "submit",});

Why are the field db and its value "dm2" recognized as illegal in the
first variant?
Thank you for your kind help.

Sebastian