diff --git a/lib/omniauth.rb b/lib/omniauth.rb
index a94d975b1..13dc9c255 100644
--- a/lib/omniauth.rb
+++ b/lib/omniauth.rb
@@ -84,13 +84,15 @@ def before_request_phase(&block)
 
     def add_mock(provider, mock = {})
       # Stringify keys recursively one level.
+      new_mock = {}
       mock.keys.each do |key|
-        mock[key.to_s] = mock.delete(key)
+        new_mock[key.to_s] = mock[key]
       end
-      mock.each_pair do |_key, val|
+      new_mock.each_pair do |_key, val|
         if val.is_a? Hash
+          new_val = {}
           val.keys.each do |subkey|
-            val[subkey.to_s] = val.delete(subkey)
+            new_val[subkey.to_s] = val[subkey]
           end
         else
           next
@@ -98,7 +100,7 @@ def add_mock(provider, mock = {})
       end
 
       # Merge with the default mock and ensure provider is correct.
-      mock = mock_auth[:default].dup.merge(mock)
+      mock = mock_auth[:default].dup.merge(new_mock)
       mock['provider'] = provider.to_s
 
       # Add it to the mocks.
diff --git a/spec/omniauth_spec.rb b/spec/omniauth_spec.rb
index 90b9db1d8..33078b801 100644
--- a/spec/omniauth_spec.rb
+++ b/spec/omniauth_spec.rb
@@ -110,6 +110,21 @@ class ExampleStrategy
         end
       end
     end
+
+    specify '.add_mock does not change the hash inputed' do
+      @hash_inputed = {
+        :uid => '12345',
+        :info => {:name => 'Joe', :email => 'joe@example.com'}
+      }
+      @original_hash = {
+        :uid => '12345',
+        :info => {:name => 'Joe', :email => 'joe@example.com'}
+      }
+
+      OmniAuth.config.add_mock(:facebook, @hash_inputed)
+
+      expect(@hash_inputed).to be == @original_hash
+    end
   end
 
   describe '.logger' do