React-Redux - No reducer provided for key "coins" : <p>Not sure why I'm getting the following errors.</p>

<p>I'm just setting up my store, actions and reducers, I haven't called dispatch on anything yet.</p>

<h3>Expected</h3>

<p>App runs fine, Redux state is not updated</p>

<h3>Results</h3>

<p><a href="https://i.stack.imgur.com/iGupu.png" rel="noreferrer"><img src="https://i.stack.imgur.com/iGupu.png" alt="enter image description here"></a></p>

src/index.js

<pre><code>import React from 'react'
import ReactDOM from 'react-dom'

import { createStore, applyMiddleware, compose } from 'redux'
import { Provider } from 'react-redux'
import thunk from 'redux-thunk'
import reducer from './reducer'

import App from './App'
import css from './coinhover.scss'

const element = document.getElementById('coinhover');

const store = createStore(reducer, compose(
    applyMiddleware(thunk),
    window.__REDUX_DEVTOOLS_EXTENSION__ &amp;&amp; window.__REDUX_DEVTOOLS_EXTENSION__()
));

ReactDOM.render(
    &lt;Provider store={ store }&gt;
        &lt;App /&gt;
    &lt;/Provider&gt;, element);
</code></pre>

src/reducer/index.js

<pre><code>import { combineReducers } from 'redux'
import { coins } from './coins'

export default combineReducers({
    coins
});
</code></pre>

src/reducer/actions/coins.js

<pre><code>import * as api from '../../services/api'
import { storage, addToPortfolio } from '../../services/coinFactory'

export const ADD_COIN = 'ADD_COIN'

export function getCoin(coin) {
    return dispatch =&gt; {
        api.getCoin(coin)
            .then((res_coin)  =&gt; addToPortfolio(res_coin))
            .then((portfolio) =&gt; dispatch(updatePortfolio(portfolio)));
    }
}

export function updatePortfolio(portfolio) {
    return {
        type: ADD_COIN,
        portfolio
    }
}
</code></pre>

finally src/reducer/coins/index.js

<pre><code>import { ADD_COIN } from './actions'

const initialState = [];

export default (state = initialState, action) =&gt; {
    switch(action.type) {
        case ADD_COIN:
            return action.portfolio;
        default:
            return state;
    }
}
</code></pre>
