ActiveRecord::StatementInvalid in FeedController#index

PG::SyntaxError: ERROR: syntax error at or near "INDEX" LINE 1: ...ted_at, entry_id, field1, field2 FROM feeds FORCE INDEX (ind... ^ : SELECT created_at, entry_id, field1, field2 FROM feeds FORCE INDEX (index_feeds_on_channel_id_and_created_at) WHERE "feeds"."channel_id" = 2 AND ("feeds"."created_at" BETWEEN '2015-07-15 14:24:18.760865' AND '2015-07-16 14:24:18.775319') ORDER BY created_at desc LIMIT 100

Extracted source (around line #114):
112
113
114
115
116
117
          
.order('created_at desc')
.limit(@limit)
.load
# sort properly
@feeds.reverse!

Rails.root: /home/dirk/thingspeak

Application Trace | Framework Trace | Full Trace
app/models/feed_factory.rb:114:in `get_feeds'
app/models/feed_factory.rb:54:in `get_output_feeds'
app/controllers/feed_controller.rb:33:in `index'

Request

Parameters:

{"key"=>"Q0F5CG24FMYJKYWP",
 "channel_id"=>"2",
 "format"=>"json"}

Response

Headers:

None