diff --git a/lib/mutant/mutator/node/named_value/constant_assignment.rb b/lib/mutant/mutator/node/named_value/constant_assignment.rb
index 2b9474f9..005105d3 100644
--- a/lib/mutant/mutator/node/named_value/constant_assignment.rb
+++ b/lib/mutant/mutator/node/named_value/constant_assignment.rb
@@ -17,7 +17,7 @@ module Mutant
           # @return [undefined]
           def dispatch
             mutate_name
-            emit_value_mutations
+            emit_value_mutations if value
             emit_remove_const
           end
 
diff --git a/meta/or_asgn.rb b/meta/or_asgn.rb
index 5660203f..4fd3d6f7 100644
--- a/meta/or_asgn.rb
+++ b/meta/or_asgn.rb
@@ -21,6 +21,12 @@ Mutant::Meta::Example.add do
   mutation '@a ||= 2'
 end
 
+Mutant::Meta::Example.add do
+  source 'Foo ||= nil'
+
+  singleton_mutations
+end
+
 Mutant::Meta::Example.add do
   source '@a ||= self.bar'
 
diff --git a/spec/integrations.yml b/spec/integrations.yml
index 20356014..39c00a28 100644
--- a/spec/integrations.yml
+++ b/spec/integrations.yml
@@ -1,18 +1,17 @@
 ---
 - name: rubyspec
   namespace: Rubyspec
-  repo_uri: 'https://github.com/rubyspec/rubyspec.git'
+  repo_uri: 'https://github.com/ruby/rubyspec.git'
   mutation_coverage: false
   mutation_generation: true
   expect_coverage: 0 # not run
   exclude:
     # Binary encoded source subjected to limitations see README of unparser
     - core/array/pack/{b,h,u}_spec.rb
-    - language/versions/*1.8*
+    - language/regexp/escapes_spec.rb
     - core/array/pack/shared/float.rb
     - core/array/pack/shared/integer.rb
     - core/array/pack/{c,m,w}_spec.rb
-    - core/regexp/shared/new.rb
     - core/regexp/shared/quote.rb
     - core/encoding/compatible_spec.rb
     - core/io/readpartial_spec.rb
diff --git a/spec/support/corpus.rb b/spec/support/corpus.rb
index 3dee8e9d..6306e816 100644
--- a/spec/support/corpus.rb
+++ b/spec/support/corpus.rb
@@ -10,10 +10,18 @@ module MutantSpec
   #
   # rubocop:disable MethodLength
   module Corpus
+    TMP                 = ROOT.join('tmp').freeze
+    EXCLUDE_GLOB_FORMAT = '{%s}'.freeze
+    RUBY_GLOB_PATTERN   = '**/*.rb'.freeze
+
+    # Not in the docs. Number from chatting with their support.
+    # 2 processors allocated per container, 4 processes works well.
+    CIRCLE_CI_CONTAINER_PROCESSES = 4
+
+    private_constant(*constants(false))
+
     # Project under corpus test
     # rubocop:disable ClassLength
-    TMP = ROOT.join('tmp').freeze
-
     class Project
       MUTEX = Mutex.new
       include Adamantium, Anima.new(
@@ -58,18 +66,16 @@ module MutantSpec
       #
       # @raise [Exception]
       #   otherwise
-      #
-      # rubocop:disable AbcSize
       def verify_mutation_generation
         checkout
         start = Time.now
-        paths = Pathname.glob(repo_path.join('**/*.rb')).sort_by(&:size).reverse
+
         options = {
           finish:       method(:finish),
           start:        method(:start),
           in_processes: parallel_processes
         }
-        total = Parallel.map(paths, options) do |path|
+        total = Parallel.map(effective_ruby_paths, options) do |path|
           count = 0
           node =
             begin
@@ -134,15 +140,31 @@ module MutantSpec
         system(%w[bundle])
       end
 
-      # Not in the docs. Number from chatting with their support.
-      CIRCLE_CI_CONTAINER_PROCESSES = 2
+      # The effective ruby file paths
+      #
+      # @return [Array<Pathname>]
+      def effective_ruby_paths
+        paths = Pathname
+          .glob(repo_path.join(RUBY_GLOB_PATTERN))
+          .sort_by(&:size)
+          .reverse
+
+        paths - excluded_paths
+      end
+
+      # The excluded file paths
+      #
+      # @return [Array<Pathname>]
+      def excluded_paths
+        Pathname.glob(repo_path.join(EXCLUDE_GLOB_FORMAT % exclude.join(',')))
+      end
 
       # Number of parallel processes to use
       #
       # @return [Fixnum]
       def parallel_processes
-        if ENV['CI']
-          Mutant::Config::DEFAULT.jobs
+        if ENV.key?('CI')
+          CIRCLE_CI_CONTAINER_PROCESSES
         else
           Parallel.processor_count
         end