ActionWebService API ignoring multiple return values?

I'm using ActionWebService and defining the following API method:

api_method(:validate,
             :expects => [{:id => :int}, {:value => :string}],
             :returns => [{:valid => :boolean}, {:error_message
=> :string}])

if I implement this method as follows:

def validate(id, value)
  return true, 'no error'
end

I get the following error when I try to invoke it:

undefined method `remote_addr' for
#<ActionWebService::Protocol::SimpleActionPackRequest:0x34bdd60>

It seems to be expecting only one return value, with type boolean.

When I look at the AWS code, it seems to be written this way (this is
lines 36-39 in casting.rb for AWS 1.2.3):

def cast_returns(api_method, return_value) # :nodoc:
  return nil if api_method.returns.nil?
  cast(return_value, api_method.returns[0])
end

Am I doing something wrong? Can't AWS have more than one return value?
What should that look like in the implementation?

Thanks in advance.

Colleen