Redmine

I am writing a redmine plugin to do some data processing. I get the following errors when adding new record. How can I solve the issue ? Thanks

Error:

ActionView::Template::Error (can't convert Symbol into String):
    1: <%= f.label :name, l(:field_name) %>
    2: <%= f.text_field :name %>
    3:
    4: <%= f.label :fullname, l(:field_fullname) %>
  lib/redmine/views/labelled_form_builder.rb:39:in `delete'
  lib/redmine/views/labelled_form_builder.rb:39:in `label_for_field'
  lib/redmine/views/labelled_form_builder.rb:31:in `label'
  app/helpers/application_helper.rb:950:in `labelled_form_for'

Controller:

class ParticipantsController < ApplicationController
...

  def new
    @participant = Participant.new
  end

  def create
    @participant = Participant.new(params[:participant])
    if @participant.save
      flash[:notice] = l(:notice_new_participant_was_created)
      redirect_to :action => :index, :project_id => @project
    else
      render :action => :new
    end
  end

new.html.erb

<% html_title l(:label_participant_title) -%>
<div class="contextual">
    <%= link_to(l(:label_back_to_index), {:controller => 'participants', :action => 'index', :project_id => @project}) %>
</div>
<h2><%= l(:label_new_participant_title) %></h2>

<%= labelled_form_for @participant, :url => participants_path(@project) do |f| %>
<%= error_messages_for 'participant' %>
    <%= render :partial => 'form', :locals => { :f => f } %>
    <%= f.submit l(:button_create) %>
<% end %>

_form.html.erb

<%= f.label :name, l(:field_name) %>
<%= f.text_field :name %>

<%= f.label :fullname, l(:field_fullname) %>
<%= f.text_field :fullname %>