diff --git a/spec/core/marshal/dump_spec.rb b/spec/core/marshal/dump_spec.rb
index dcf1719f31..f7ce367bf3 100644
--- a/spec/core/marshal/dump_spec.rb
+++ b/spec/core/marshal/dump_spec.rb
@@ -144,10 +144,8 @@
 
     it "raises a TypeError if _dump returns a non-string" do
       m = mock("marshaled")
-      NATFIXME 'raises a TypeError if _dump returns a non-string', exception: SpecFailedException do
-        m.should_receive(:_dump).and_return(0)
-        -> { Marshal.dump(m) }.should raise_error(TypeError)
-      end
+      m.should_receive(:_dump).and_return(0)
+      -> { Marshal.dump(m) }.should raise_error(TypeError)
     end
 
     it "raises TypeError if an Object is an instance of an anonymous class" do
diff --git a/src/marshal.rb b/src/marshal.rb
index 528309898d..2d36e1f1b1 100644
--- a/src/marshal.rb
+++ b/src/marshal.rb
@@ -257,6 +257,7 @@ def write_user_marshaled_object_without_allocate(value)
       write_char('u')
       write(value.class.to_s.to_sym)
       dump = value.send(:_dump, -1)
+      raise TypeError, '_dump() must return string' unless dump.is_a?(String)
       write_integer_bytes(dump.size)
       write_bytes(value.send(:_dump, -1))
     end