This error in our Rails application drove me crazy. The error was like this:

undefined method xxxxxx' for #<Hash:0x000000001234>

The problem was that i was calling a method like this: a_method(var: var)

And the method definition was like def a_method(var: var) which resulted in something called variable shadowing and caused clash between variable passed in the method and local variable being defined.

The fix was to change method definition to def a_method(var:)

Still learning Ruby.