Assertion fails "FAILED - null/empty response body"

I'm using SOAPUI NG (Ready API 1.7) on Windows 10

I'm trying to use a Mesage Content Assertion to  check that a web service returns the correct error code.

I've done the same thing many times before in the old SoapUI (5.1.2) and it worked fine.
How in SoapUI NG, the same thing fails

After having run the WS and it has produced the expected response.

I add the assertion, Select the WS in the Source drop down box
Select Message Content Assertion
Click Add and tick the boxes for the value I want to check

Save and then run the test case again

The assertion then fails with the message "FAILED - null/empty response body"

Has something changed between SoapUI Pro & SoapUI NG?