diff --git a/lib/sprockets-vendor_gems.rb b/lib/sprockets-vendor_gems.rb
index 9f986a9..9ef9faf 100644
--- a/lib/sprockets-vendor_gems.rb
+++ b/lib/sprockets-vendor_gems.rb
@@ -9,7 +9,7 @@ module ::Sprockets
 
     paths = []
 
-    Gem::Specification.each do |gemspec|
+    GemEnvironment.each do |gemspec|
       options[:paths].product(for_types).each do |base_dir, type|
         path = File.join(gemspec.gem_dir, base_dir, "assets", type.to_s)
 
@@ -27,5 +27,19 @@ module ::Sprockets
       Sprockets.find_gem_vendor_paths.each { |path| append_path(path) }
     end
   end
+
+  class GemEnvironment
+    extend Enumerable
+
+    def self.gemspecs
+      Gem.loaded_specs.map(&:last)
+    end
+
+    def self.each(&block)
+      return enum_for(:each) unless block_given?
+
+      gemspecs.each {|spec| yield spec }
+    end
+  end
 end
 
diff --git a/spec/sprockets-vendor_gems_spec.rb b/spec/sprockets-vendor_gems_spec.rb
index 3d39330..28a480a 100644
--- a/spec/sprockets-vendor_gems_spec.rb
+++ b/spec/sprockets-vendor_gems_spec.rb
@@ -17,7 +17,7 @@ describe Sprockets do
     before do
       paths.each { |path| FileUtils.mkdir_p path }
 
-      Gem::Specification.stubs(:each).yields(gemspec)
+      Sprockets::GemEnvironment.stubs(:each).yields(gemspec)
     end
 
     subject { described_class.find_gem_vendor_paths }