xp20:compare function in BPEL

When you use xp20:compare function you get below error.

xp20:compare($in.payload/tns:taskAssignedMessage/task:task/task:sca/task:compositeName, 'HumantaskPJ1')

Non Recoverable System Fault :

ORAMED-01102:[Filter Expression Metadata Error]Error evaluating filter expression, "xp20:compare($in.payload/tns:taskAssignedMessage/task:task/task:sca/task:compositeName, 'HumantaskPJ1')".Root-cause: "java.lang.ClassCastException: oracle.xml.parser.v2.XMLElement cannot be cast to java.lang.String". Possible Fix:Ensure that the filter expression metadata is valid, otherwise Contact Oracle Support Services.

to fix the problem use 

xp20:compare(string($in.payload/tns:taskAssignedMessage/task:task/task:sca/task:compositeName), 'HumantaskPJ1')