Index: actionpack/lib/action_controller/assertions/response_assertions.rb
===================================================================
--- actionpack/lib/action_controller/assertions/response_assertions.rb	(revision 8975)
+++ actionpack/lib/action_controller/assertions/response_assertions.rb	(working copy)
@@ -33,8 +33,8 @@
           elsif type.is_a?(Symbol) && @response.response_code == ActionController::StatusCodes::SYMBOL_TO_STATUS_CODE[type]
             assert_block("") { true } # to count the assertion
           else
-            if @response.error?
-              assert_block(build_message(message, "Expected response to be a <?>, but was <?>\n<?>", type, @response.response_code, @response.template.instance_variable_get(:@exception).message)) { false }
+            if @response.error? && exception = @response.template.instance_variable_get(:@exception)
+              assert_block(build_message(message, "Expected response to be a <?>, but was <?>\n<?>", type, @response.response_code, exception.message)) { false }
             else
               assert_block(build_message(message, "Expected response to be a <?>, but was <?>", type, @response.response_code)) { false }
             end