TypeError: Font.isLoaded is not a function, (in 'Font.isLoaded(fontName)', 'Font.isLoaded' is undefined) in Expo

I'm building a React Native app using Expo 31.0.4 and Expo Vector icons. When I'm using Ionicons to build the TabBar, an error occurs.

import { Ionicons } from '@expo/vector-icons';

const Main = TabNavigator({
    Settings: {
        screen: ProfileStack,
        navigationOptions: ({navigation}) => ({
            tabBarIcon: ({tintColor}) => (<Ionicons name="ios-cog" size={25} color={tintColor}/>)
        })
    },
    Deals: {
        screen: DealStack,
        navigationOptions: ({navigation}) => ({
            tabBarIcon: ({tintColor}) => (<Ionicons name="ios-list-box" size={25} color={tintColor}/>),
        })
    },
    Checkin: {
        screen: BarcodeScannerScreen,
        navigationOptions: ({navigation}) => ({
            tabBarIcon: ({tintColor}) => (<Ionicons name="ios-camera" size={27} color={tintColor}/>)
        })
    },

Please help me, thanks.