Groovy MissingMethodException on closure

Question

I have the following Groovy closure:

class Widget {
    def determineDatastore = { ServiceInstance serviceInstance, String datastoreFolderPath ->
        def datastoreFolder = serviceInstance.searchIndex.findByInventoryPath(datastoreFolderPath) as Folder
        def datastores = datastoreFolder.childEntity.findAll {!it.name.contains("templates")}
        return datastores.max {
            Datastore datastore -> datastore.info.freeSpace
        }
    }   

    static void main(String[] args) {
        Widget w = new Widget()
        w.determineDatastore(ServiceInstance.get(), DataStoreFolderPath.get())
    }
}

When I run this I am getting:

groovy.lang.MissingMethodException: No signature of method: com.me.myapp.Widget$_closure1_closure3.doCall() is applicable for argument types: (com.vmware.vim25.mo.Folder) values: [Folder:group-s38651 @ https://dev-vcenter.myorg.com/sdk]
Possible solutions: doCall(com.vmware.vim25.mo.Datastore), findAll(), findAll(), isCase(java.lang.Object), isCase(java.lang.Object)
    at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:264)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:909)
    at groovy.lang.Closure.call(Closure.java:423)
    at groovy.lang.Closure.call(Closure.java:439)

What is going on here?