calling method with List in signature

I have a java class with a method that takes two arguments.  One is a class I 
define and the other is a List interface.  The call to the method from groovy 
works if the List is empty, but fails if the List has elements in it.  I get an 
error that the second argument is the wrong class and the class type is the 
first element of the List.  How do I tell it to pass the List to the method?
 
Vector blockList = new Vector(); 
blockList.add( salesRollupBlock );
yyy.populateCategoryRollupFromBlocks( catRollupBlock, (List)blockList );
 
 
groovy.lang.GroovyRuntimeException: failed to invoke method: groovy.lang.MetaMet
hod@14e5d57[name: populateCategoryRollupFromBlocks params: [class com.perfretail
.k2v1.trackers.sales.CategoryRollupBlock, interface java.util.List] returns: voi
d owner: class com.perfretail.k2v1.envoys.sales.SalesTotalEnvoy] on: com.perfret
ail.k2v1.envoys.sales.SalesTotalEnvoy@1ad9b0f with arguments: [com.perfretail.k2
v1.trackers.sales.CategoryRollupBlock@15d533d, [com.perfretail.k2v1.trackers.sal
es.SalesRollupBlock@178b0f9]] reason: java.lang.ClassCastException
        at groovy.lang.MetaClass.doMethodInvoke(MetaClass.java:1383)
        at groovy.lang.MetaClass.invokeMethod(MetaClass.java:316)
        at org.codehaus.groovy.runtime.Invoker.invokeMethod(Invoker.java:143)
        at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.
java:106)
        at script1097528528832.run(script1097528528832.groovy:78)
        at gjdk.script1097528528832_GroovyReflector.invoke(script1097528528832_G
roovyReflector.java)
        at groovy.lang.MetaMethod.invoke(MetaMethod.java:110)
        at groovy.lang.MetaClass.doMethodInvoke(MetaClass.java:1372)
        at groovy.lang.MetaClass.invokeMethod(MetaClass.java:316)
        at groovy.lang.MetaClass.invokeMethod(MetaClass.java:301)
        at groovy.lang.GroovyObjectSupport.invokeMethod(GroovyObjectSupport.java
:73)
 
Randy