Exception in SoapActionInInterceptor: java.lang.StringIndexOutOfBoundsException: String index out of range: -1

Issue

I have a webservice client that is always receiving the following error when they invoke our endpoint:

08:13:42,733 WARNING [org.apache.cxf.phase.PhaseInterceptorChain] (http-/0.0.0.0:8080-41) Interceptor for {http://www.test.com/}GenericWebService has thrown exception, unwinding now: java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.lang.String.substring(String.java:1911) [rt.jar:1.7.0_55]
        at org.apache.cxf.binding.soap.interceptor.SoapActionInInterceptor.getSoapAction(SoapActionInInterceptor.java:67) [cxf-rt-bindings-soap-2.6.8.redhat-7.jar:2.6.8.redhat-7]
        at org.apache.cxf.binding.soap.interceptor.SoapActionInInterceptor.handleMessage(SoapActionInInterceptor.java:103) [cxf-rt-bindings-soap-2.6.8.redhat-7.jar:2.6.8.redhat-7]
        at org.apache.cxf.binding.soap.interceptor.SoapActionInInterceptor.handleMessage(SoapActionInInterceptor.java:44) [cxf-rt-bindings-soap-2.6.8.redhat-7.jar:2.6.8.redhat-7]
        at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:262) [cxf-api-2.6.8.redhat-7.jar:2.6.8.redhat-7]
        at org.apache.cxf.transport.ChainInitiationObserver.onMessage(ChainInitiationObserver.java:121) [cxf-api-2.6.8.redhat-7.jar:2.6.8.redhat-7]