Filtering by date is giving me a following error: "value of filter criterion for field 'Opportunity_Closed_Won_Date__c' must be of type date and should not be enclosed in quotes"

[Naoya Makino]

Hi there, when I do find by conditions like:
Salesforce::Account.find(:all, :conditions =>
['opportunity_closed_won_date__c >= ?', Date.today - 30.days])
it is giving me this error message:
ERROR at Row:1:Column:1648
value of filter criterion for field 'Opportunity_Closed_Won_Date__c'
must be of type date and should not be enclosed in quotes

I have seen a similar post like this one
http://rubyforge.org/forum/forum.php?thread_id=24181&forum_id=5048
Is this a bug? or am I doing something wrong? Please be advised

thanks