Scripting Message Content Assertion

I'm trying to modify a Message Content Assertion dynamically in a groovy script.

Assume I have a testcase with two test steps:
1. Groovy Script
2. SOAP call


The SOAP call has one Message Content Assertion. Now I try to dynamically change the first value of the assertion in the Groovy script:

wsdlTestSteps = testRunner.testCase.getTestStepsOfType( com.eviware.soapui.impl.wsdl.teststeps.WsdlTestRequestStep.class );
wsdlTestSteps.each {
 def assertion = it.getAssertionByName("Message Content Assertion");
 if (assertion) {
  assertion.getConfiguration().getElementsArray(0).setExpectedValue('some value')
 }
}

The value is changed in the xml file, but the testcase won't fail, when I try to run it.

With an XPath assertion I can easily get it to work, but then I'm forced to have a lot of XPath assertions, which increases the risk, that I forget to test some fields.

wsdlTestSteps = testRunner.testCase.getTestStepsOfType( com.eviware.soapui.impl.wsdl.teststeps.WsdlTestRequestStep.class );
wsdlTestSteps.each {
 def assertion = it.getAssertionByName("XPath Match");
 if(assertion) {
  assertion.setExpectedContent('some content');
 }
}