diff --git a/lib/rb/spec/struct_spec.rb b/lib/rb/spec/struct_spec.rb
index 5a4dc6a..8e106fa 100644
--- a/lib/rb/spec/struct_spec.rb
+++ b/lib/rb/spec/struct_spec.rb
@@ -150,4 +150,10 @@
     struct.complex = {5 => {"foo" => 1.23}}
     struct.write(prot)
   end
+
+  it "should raise an exception if presented with an unknown container" do
+    # yeah this is silly, but I'm going for code coverage here
+    struct = Foo.new
+    lambda { struct.send :write_container, nil, nil, {:type => "foo"} }.should raise_error(StandardError, "Not a container type: foo")
+  end
 end