Issue Using xp20:upper-case in Assign Activity

Strange issue with using xp20:upper-case in assign activity. Below will not work -

xp20:upper-case($inputVariable.payload/client:MessageType)

where MessageType is of type string.

It will work like this -

xp20:upper-case(string($inputVariable.payload/client:MessageType))

This issue was encountered in SOA 11.1.1.5 - bpel 2.0