Accessing vuex modules data from within a plugin - vue.js

Question

I am moving some data from the vuex store into its own module. Most of it works great, but I'm running into an issue that I can't seem to fix.
I have a plugin that I add, and that plugin also needs access to the store.
So at the top of that plugin, we import the store:
import store from '../store/store';
great - further down that plugin, I'm accessing the data of the store in a method that I expose on the service:
hasPermission(permission) {
return (store.state.authorization.permissions.indexOf(permission) >= 0);
}
Please note authorization is now a seperate module, and is no longer part of the root state object.
now the funny thing is that the above will return an error telling me indexOf is not a function.
When I add the following, however:
hasPermission(permission) {
console.log('Validating permission ' + permission);
console.log(store.state);
return (store.state.authorization.permissions.indexOf(permission) >= 0);
}
I notice that (1) the output to console is what I expect it to be, and (2), I'm not getting the error, and my menu structure dynamically builds as expected...
so I'm a bit confused to say the least...
authorization.permissions is updated each time a user authenticates, logs out, or chooses another account to work on; in these cases we fetch updated permissions from the server and commit it to the store, so we can build our menu structure based on up-to-date permissions. Works pretty good, but I'm not sure I understand why this fails to be honest.
The plugin is created as follows in the install:
Vue.prototype.$security = new Vue(
...
methods: {
hasPermission: function(permission) {
...
}
}
...
);