ArgumentError key must be 32 bytes

I create new application in localhost
Affter run rails generate controller StaticPages home help
I navigate to http://localhost:3000/static_pages/home
1 Error response .
Help me fix please ! thanks all

ArgumentError in StaticPagesController#home
key must be 32 bytes
Extracted source (around line #72):

70 cipher = new_cipher
71 cipher.encrypt
72 cipher.key = @secret
73
74 # Rely on OpenSSL for the initialization vector
75 iv = cipher.random_iv

Rails.root: /home/nhattrung/Working/sample_app