Schema Compliance - GUID Pattern?

When performing a schema compliance assertion, I get the following failure message:

[Schema Compliance] string value '21b9dead-ea5b-5f05-740f-015f8e118056' does not match pattern for guid in namespace http://microsoft.com/wsdl/types/

What is the pattern that soapUI is looking for? From what I can see, the GUID pattern matches the definition. Any help? It's hard to use the schema compliance assertion if I'm always getting these false-failures. Is this an issue with soapUI? Is there a workaround?

Thanks!