Google AppEngine does not like leftshift

I ran into an issue last night where Google AppEngine wouldn’t persist my entities. If I create an entity using the parameters provided by Grails; then the Save operations would fail with the following error

Uncaught exception from servlet org.codehaus.groovy.runtime.InvokerInvocationException: groovy.lang.MissingMethodException: No signature of method: java.lang.StringBuilder.leftShift() is applicable for argument types: (org.codehaus.groovy.runtime.GStringImpl) values: [( = ?1 )] at com.studentsonly.grails.plugins.uiperformance.CacheFilter.doFilter( at at at at at at at$EvaluationRuntime$6.handleBlockingRequest( at$EvaluationRuntime$6.handleBlockingRequest( at at at$ ...

The issue is described here. I tried their solution and it works fine for me.