diff --git a/spec/unit/mutant/actor/env_spec.rb b/spec/unit/mutant/actor/env_spec.rb
index f3129017..cb0d98dd 100644
--- a/spec/unit/mutant/actor/env_spec.rb
+++ b/spec/unit/mutant/actor/env_spec.rb
@@ -1,6 +1,6 @@
 RSpec.describe Mutant::Actor::Env do
-  let(:thread)      { instance_double(Thread)                 }
-  let(:thread_root) { instance_double(Thread.singleton_class) }
+  let(:thread)      { instance_double(Thread) }
+  let(:thread_root) { class_double(Thread)    }
 
   let(:object) { described_class.new(thread_root) }
 
diff --git a/spec/unit/mutant/env_spec.rb b/spec/unit/mutant/env_spec.rb
index 21525531..79548d63 100644
--- a/spec/unit/mutant/env_spec.rb
+++ b/spec/unit/mutant/env_spec.rb
@@ -13,18 +13,15 @@ RSpec.describe Mutant::Env do
       )
     end
 
-    let(:integration)       { instance_double(Mutant::Integration) }
-    let(:wrapped_node)      { instance_double(Parser::AST::Node)   }
-    let(:context)           { instance_double(Mutant::Context)     }
-    let(:test_a)            { instance_double(Mutant::Test)        }
-    let(:test_b)            { instance_double(Mutant::Test)        }
-    let(:tests)             { [test_a, test_b]                     }
-    let(:selector)          { instance_double(Mutant::Selector)    }
-    let(:integration_class) { Mutant::Integration::Null            }
-
-    let(:isolation) do
-      instance_double(Mutant::Isolation::Fork.singleton_class)
-    end
+    let(:integration)       { instance_double(Mutant::Integration)  }
+    let(:wrapped_node)      { instance_double(Parser::AST::Node)    }
+    let(:context)           { instance_double(Mutant::Context)      }
+    let(:test_a)            { instance_double(Mutant::Test)         }
+    let(:test_b)            { instance_double(Mutant::Test)         }
+    let(:tests)             { [test_a, test_b]                      }
+    let(:selector)          { instance_double(Mutant::Selector)     }
+    let(:integration_class) { Mutant::Integration::Null             }
+    let(:isolation)         { class_double(Mutant::Isolation::Fork) }
 
     let(:mutation) do
       instance_double(
@@ -37,7 +34,7 @@ RSpec.describe Mutant::Env do
       Mutant::Config::DEFAULT.with(
         isolation:   isolation,
         integration: integration_class,
-        kernel:      instance_double(Kernel.singleton_class)
+        kernel:      class_double(Kernel)
       )
     end
 
diff --git a/spec/unit/mutant/loader_spec.rb b/spec/unit/mutant/loader_spec.rb
index 1d550553..d6290d18 100644
--- a/spec/unit/mutant/loader_spec.rb
+++ b/spec/unit/mutant/loader_spec.rb
@@ -11,7 +11,7 @@ RSpec.describe Mutant::Loader, '.call' do
   let(:path)     { instance_double(Pathname, to_s: path_str) }
   let(:path_str) { instance_double(String)                   }
   let(:line)     { instance_double(Fixnum)                   }
-  let(:kernel)   { instance_double(Kernel.singleton_class)   }
+  let(:kernel)   { class_double(Kernel)                      }
   let(:binding)  { instance_double(Binding)                  }
   let(:source)   { instance_double(String)                   }
   let(:node)     { instance_double(Parser::AST::Node)        }
diff --git a/spec/unit/mutant/repository/diff_spec.rb b/spec/unit/mutant/repository/diff_spec.rb
index f821078f..eef3760f 100644
--- a/spec/unit/mutant/repository/diff_spec.rb
+++ b/spec/unit/mutant/repository/diff_spec.rb
@@ -17,12 +17,12 @@ describe Mutant::Repository::Diff do
       )
     end
 
-    let(:pathname)   { instance_double(Pathname.singleton_class, pwd: pwd) }
-    let(:open3)      { instance_double(Open3.singleton_class)              }
-    let(:kernel)     { instance_double(Kernel.singleton_class)             }
-    let(:pwd)        { Pathname.new('/foo')                                }
-    let(:path)       { Pathname.new('/foo/bar.rb')                         }
-    let(:line_range) { 1..2                                                }
+    let(:pathname)   { class_double(Pathname, pwd: pwd) }
+    let(:open3)      { class_double(Open3)              }
+    let(:kernel)     { class_double(Kernel)             }
+    let(:pwd)        { Pathname.new('/foo')             }
+    let(:path)       { Pathname.new('/foo/bar.rb')      }
+    let(:line_range) { 1..2                             }
 
     subject { object.touches?(path, line_range) }
 
diff --git a/spec/unit/mutant/runner_spec.rb b/spec/unit/mutant/runner_spec.rb
index bd9a584f..b827a097 100644
--- a/spec/unit/mutant/runner_spec.rb
+++ b/spec/unit/mutant/runner_spec.rb
@@ -6,7 +6,7 @@ RSpec.describe Mutant::Runner do
     let(:delay)       { instance_double(Float)                          }
     let(:env_result)  { instance_double(Mutant::Result::Env)            }
     let(:actor_env)   { instance_double(Mutant::Actor::Env)             }
-    let(:kernel)      { instance_double(Kernel.singleton_class)         }
+    let(:kernel)      { class_double(Kernel)                            }
     let(:sleep)       { instance_double(Method)                         }
 
     let(:env) do
diff --git a/spec/unit/mutant/zombifier_spec.rb b/spec/unit/mutant/zombifier_spec.rb
index 7dd5b3ef..98621a1a 100644
--- a/spec/unit/mutant/zombifier_spec.rb
+++ b/spec/unit/mutant/zombifier_spec.rb
@@ -1,9 +1,6 @@
 RSpec.describe Mutant::Zombifier do
   let(:root_require) { Pathname.new('project') }
-
-  let(:pathname) do
-    instance_double(::Pathname.singleton_class)
-  end
+  let(:pathname)     { class_double(Pathname)  }
 
   let(:require_highjack) do
     lambda do |block|