Error! when we print a json value as a string i am getting error

Summary

how to print a json value in OIC

Content

I want to calculate signature using payload(json) which is coming in OIC flow.

when i try to print a sample json value i am getting below error

Invalid XPath expression. an error occurs while parsing XPath expression "{"key":"value"}". The XPath expression was invalid; the reason was: Unknown expression at EOF: "{"key":"value"}". Check the detailed root cause described in the exception message text and verify that the XPath expression named in the error message is correct. The XPath expression is defined in the BPEL process.

 

I hard-cord a json value in following pattern