After a trivial change to taglib, you get: Grails Runtime Exception Error Details Message: object is not an instance of declaring class Caused by: object is not an instance of declaring class Class: index.gsp At Line: [58] Code Snippet: 58: 59: Stack Trace java.lang.IllegalArgumentException: object is not an instance of declaring class at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at org.codehaus.groovy.runtime.ReflectionMetaMethod.invoke(ReflectionMetaMethod.java:69) at org.codehaus.groovy.runtime.MetaClassHelper.doMethodInvoke(MetaClassHelper.java:713) at groovy.lang.MetaClassImpl.getProperty(MetaClassImpl.java:882) at groovy.lang.MetaClassImpl.getProperty(MetaClassImpl.java:2090) at groovy.lang.DelegatingMetaClass.getProperty(DelegatingMetaClass.java:97) at org.codehaus.groovy.grails.web.metaclass.TagLibMetaClass.getProperty(TagLibMetaClass.java:95) at gjdk.org.codehaus.groovy.grails.web.metaclass.TagLibMetaClass_GroovyReflector.invoke(Unknown Source) at groovy.lang.MetaMethod.invoke(MetaMethod.java:115) at org.codehaus.groovy.runtime.MetaClassHelper.doMethodInvoke(MetaClassHelper.java:713) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:559) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:449) at