diff --git a/Gemfile b/Gemfile
index bf05881..5562f2c 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,5 +6,10 @@ gemspec
 gem "jekyll", "~> #{ENV["JEKYLL_VERSION"]}" if ENV["JEKYLL_VERSION"]
 
 group :test do
+  # Temporarily lock rspec dependencies to last known versions with
+  # which tests passed successfully on CI
+  gem "rspec-expectations", "3.8.4"
+  gem "rspec-mocks", "3.8.1"
+
   gem "webmock", "~> 2.0"
 end
diff --git a/jekyll-github-metadata.gemspec b/jekyll-github-metadata.gemspec
index 629bd3c..babaac3 100644
--- a/jekyll-github-metadata.gemspec
+++ b/jekyll-github-metadata.gemspec
@@ -23,6 +23,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "netrc"
   spec.add_development_dependency "pry"
   spec.add_development_dependency "rake"
-  spec.add_development_dependency "rspec"
+  spec.add_development_dependency "rspec", "~> 3.8.0"
   spec.add_development_dependency "rubocop-jekyll", "~> 0.5.0"
 end
diff --git a/lib/jekyll-github-metadata/metadata_drop.rb b/lib/jekyll-github-metadata/metadata_drop.rb
index b870689..83e106a 100644
--- a/lib/jekyll-github-metadata/metadata_drop.rb
+++ b/lib/jekyll-github-metadata/metadata_drop.rb
@@ -14,7 +14,7 @@ class MetadataDrop < Jekyll::Drops::Drop
       alias_method :invoke_drop, :[]
       def key?(key)
         return false if key.nil?
-        return true if self.class.mutable? && @mutations.key?(key)
+        return true if self.class.mutable? && mutations.key?(key)
 
         respond_to?(key) || fallback_data.key?(key)
       end
@@ -95,6 +95,10 @@ def build_revision
       def fallback_data
         @fallback_data ||= {}
       end
+
+      def mutations
+        @mutations ||= {}
+      end
     end
   end
 end