method_missing Error when using Engine gemspec add_dependency

[nugenius]

An error is produced when using validates_formatting_of with a Rails Engine by including it in the [engine].gemspec file.

# Describe your gem and declare its dependencies:
Gem::Specification.new do |s|
...
  s.add_dependency 'validates_formatting_of'
...
end

Produces

method_missing': undefined methodvalidates_formatting_of' for ....

If the gem is included through the Gemfile all runs correctly.

This is important because the dependencies for engines are supposed to be handled as if the engine is a gem and thus you should use the [engine].gemspec file.

[mdespuits]

Could you include the full backtrace?