Although rootState.user.user_data returns an object, rootState.user.user_data.uid returns null

I have a problem. Imagine this mutation:

SET_USER(state, payload) {
            console.log("SET USER COMMIT")
            Vue.set(state.user_data, 'uid', payload.uid || payload._id)
            state.user_data.display_name = payload.display_name
            state.user_data.email = payload.email
            state.user_data.registered_on = payload.registered_on
            state.user_data.confirmed = payload.confirmed
            state.user_data.interacted_tags = payload.interacted_tags

        },

And now, imagine this accessing to the rootState:

 test({rootState}, uid) {
            console.log(uid)
            console.log("uid", rootState.user.user_data.uid)
        }

Although user_data.uid is NOT null, and has a value which I see using getters (which also return null in actions), this console.log returns a null value. What could be wrong?

Here's an image of the user_data in the console: