diff --git a/spec/ruby/core/env/assoc_spec.rb b/spec/ruby/core/env/assoc_spec.rb
index 853eca79a5..e544116967 100644
--- a/spec/ruby/core/env/assoc_spec.rb
+++ b/spec/ruby/core/env/assoc_spec.rb
@@ -1,23 +1,30 @@
 require_relative '../../spec_helper'
 
 describe "ENV.assoc" do
+  before :each do
+    reserve_names("foo")
+  end
+
   after :each do
-    ENV.delete("foo")
+    release_names
   end
 
-  it "returns an array of the key and value of the environment variable with the given key" do
+  it "returns an array of the name and value of the environment variable with the given name" do
     ENV["foo"] = "bar"
     ENV.assoc("foo").should == ["foo", "bar"]
   end
 
-  it "returns nil if no environment variable with the given key exists" do
+  it "returns nil if no environment variable with the given name exists" do
     ENV.assoc("foo").should == nil
   end
 
-  it "returns the key element coerced with #to_str" do
+  it "coerces a non-String name by calling :to_str" do
     ENV["foo"] = "bar"
-    k = mock('key')
-    k.should_receive(:to_str).and_return("foo")
-    ENV.assoc(k).should == ["foo", "bar"]
+    mock_object = mock_to_str(:foo)
+    ENV.assoc(mock_object).should == ["foo", "bar"]
+  end
+
+  it "raises TypeError if the argument is not a String and does not respond to :to_str" do
+    -> { ENV.assoc(Object.new) }.should raise_error(TypeError)
   end
 end
diff --git a/spec/ruby/spec_helper.rb b/spec/ruby/spec_helper.rb
index c38965d3c5..408efb2e23 100644
--- a/spec/ruby/spec_helper.rb
+++ b/spec/ruby/spec_helper.rb
@@ -3,6 +3,28 @@
 dir = "fixtures/code"
 CODE_LOADING_DIR = use_realpath ? File.realpath(dir, root) : File.expand_path(dir, root)
 
+# Reserve names.
+def reserve_names(*names)
+  @reserved = names
+  @reserved.each  do |name|
+    fail "Name #{name} is already in use" if ENV.include?(name)
+  end
+end
+
+# Release reserved names.
+def release_names
+  @reserved.each do |name|
+    ENV.delete(name)
+  end
+end
+
+# Mock object for calling to_str.
+def mock_to_str(s)
+  mock_object = mock('name')
+  mock_object.should_receive(:to_str).and_return(s.to_s)
+  mock_object
+end
+
 # Enable Thread.report_on_exception by default to catch thread errors earlier
 if Thread.respond_to? :report_on_exception=
   Thread.report_on_exception = true