Input-event seems not to be available on a vuetify checkbox?

I want to invoke a methode, when the user selects one of the checkboxes which I am rendering via v-for. They all are bound to one variable via v-model. Though the problem is that the input doesn't even seem to be available on v-checkbox. And the following code doesn't do anything:

<template>
    <v-container>
        <v-layout row>
                <v-flex xs12>
                <v-layout justify-center>
                    <v-form ref="form" lazy-validation>
                        <v-checkbox
                        v-for="(answer, index) in questions[actualIndex].answers"
                        :key="index"
                        :ref="'check'+index"
                        :label="answer.answer"
                        :value="answer.answer"
                        v-model="checkbox"
                        @input="onInput()"
                        ></v-checkbox>
                    </v-form>
                </v-layout>
            </v-flex>
            </v-layout>
    </v-container>
</template 

export default {
data() {
    return {
    checkbox: ''
        }
    },
methods: {
onInput() {
        console.log("WORKS");

    }

  }
}

However if I use the @change or @click events instead, it works perfectly. Any ideas, what I am doing wrong?