When running gem build on a new library I have been working on I started getting the following error:

[ruby_svg_light.gemspec] isn't a Gem::Specification (Rake::FileTask instead).
ERROR:  While executing gem ... (NoMethodError)
    undefined method `mark_version' for nil:NilClass

It was from a gemspec similar to that below

require 'rake/gempackagetask' 

NAME = "ruby_svg_light" 
Dir.chdir( './lib/')
require NAME
Dir.chdir( './../')

spec = Gem::Specification.new do |s|
  s.name         = NAME
  s.version      = RubySVGLight::VERSION
  s.platform     = Gem::Platform::RUBY
  s.summary      = 'Basic Helper for creating SVG Files'
  s.homepage     = 'http://amaras-tech.co.uk/software/RubySVGLight'
  s.authors      = "Morgan Prior"
  s.email        = NAME + "_gem@amaras-tech.co.uk"
  s.description  = %{A basic library for building SVG files. Not all properties of SVG are supported}
  s.files        = [Dir.glob("LICENSE.rtf")]
  s.files        += Dir.glob("examples/*")
  s.files        += Dir.glob("lib/**/*")
  s.files        += Dir.glob("spec/*") 
end
Rake::GemPackageTask.new(spec).define

The Fix was to change to this (Removing Rake::GemPackageTask):

NAME = "ruby_svg_light"
Dir.chdir( './lib/')
require NAME
Dir.chdir( './../')

Gem::Specification.new do |s|
  s.name         = NAME
  s.version      = RubySVGLight::VERSION
  s.platform     = Gem::Platform::RUBY
  s.summary      = 'Basic Helper for creating SVG Files'
  s.homepage     = 'http://amaras-tech.co.uk/software/RubySVGLight'
  s.authors      = "Morgan Prior"
  s.email        = NAME + "_gem@amaras-tech.co.uk"
  s.description  = %{A basic library for building SVG files. Not all properties of SVG are supported}
  s.files        = [Dir.glob("LICENSE.rtf")]
  s.files        += Dir.glob("examples/*")
  s.files        += Dir.glob("lib/**/*")
  s.files        += Dir.glob("spec/*")
end

It looks like the new format if for the gemspec is to return a Gem::Specification rather than run the task. I do not know when this change happened.