Error from Data_Wrap_Struct when class is inside a module

I'm playing around with writing a ruby extension in C and I'm getting  
a "wrong argument type Class (expected Class) (TypeError)" error that  
appears to be coming from my Data_Wrap_Struct call.

Parred down C code:

VALUE Library_new(VALUE klass)
{
	my_struct* lib;
	VALUE new_inst;

	// C fuction to allocate and initialize lib
	init(lib);

	// create new ruby object around C structure (Library_free defined  
but not shown)
	new_inst = Data_Wrap_Struct(klass, 0, Library_free, lib);

	return new_inst;
}

void Init_extension() {
	VALUE classLibrary = Qnil;

	classLibrary = rb_define_class("Library", rb_cObject);
	rb_define_singleton_method(classLibrary, "new", Library_new, 0);
}

And in ruby:

require 'extension'

lib = Library.new

Now, in the process of debugging and trying to put in some debug  
statements, I have discovered that by adding the line:

printf("class is a: %s\n", STR2CSTR(rb_funcall(rb_funcall 
(klass,rb_intern("class"),0),rb_intern("to_s"),0)));

just before the Data_Wrap_Struct call, then I don't get the error and  
appear to have an initialized library. Why would calling the  
equivalent of a 'self.class.to_s' change anything? I'm running "ruby  
1.8.7 (2010-12-23 patchlevel 330) [powerpc-darwin8]". Any help or  
suggestions are much appreciated.

Michael