undefined method `subregions'

[thukim]

using subregion_select(@object, :region, {priority: ['US', 'CA']}, class: 'region') in my form returns
undefined methodsubregions' for {:priority=>["US", "CA"]}:Hash`
but the below one works:

subregion_select(@object, :region, 'US', class: 'region')

[webdev36]

It is working well on my local, but I have got this error on heroku.
How can I solve this issue?