Redux Thunk doesn't seem to be registered properly

clientapp.js

import React from 'react';
import { render } from 'react-dom';
import App from '../shared/App';

// REDUX IMPORTS
import { Provider } from 'react-redux';
import { ConnectedRouter } from 'react-router-redux';

// build the redux store
const storeBuilder = require('../redux/store/configureStore');
const store = storeBuilder.default();
// use the same history as the redux middleware;
const history = storeBuilder.history;

render(
    <Provider store={store}>
        <ConnectedRouter history={history}>
            <App/>
        </ConnectedRouter>
    </Provider>,
    document.getElementById('site-wrapper')
);

configureStore.dev.js

import { createStore, applyMiddleware, compose } from 'redux'
import thunk from 'redux-thunk';
import { routerMiddleware } from 'react-router-redux';
import {createLogger} from 'redux-logger';
import rootReducer from '../reducers'

// history
import createHistory from 'history/createBrowserHistory';
const history = createHistory();

const middleware = [routerMiddleware(history), thunk, createLogger()];

const configureStore = preloadedState => {
    const store = createStore(
        rootReducer,
        preloadedState,
        compose(
            applyMiddleware(...middleware),
            window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
        )
    );

    if (module.hot) {
        // Enable Webpack hot module replacement for reducers
        module.hot.accept('../reducers', () => {
            const nextRootReducer = require('../reducers');
            store.replaceReducer(nextRootReducer)
        })
    }

    return store
};

export default configureStore;
export { history };

inventoryActions.js

import fetch from 'isomorphic-fetch';

export const INVENTORY_SEARCH_LOADING = 'INVENTORY_SEARCH_INVENTORY';
export const INVENTORY_SEARCH_RESULTS = 'INVENTORY_SEARCH_RESULTS';
export const INVENTORY_SEARCH_ERROR = 'INVENTORY_SEARCH_ERROR';

export function inventorySearchLoading(isLoading) {
    return {
        type: INVENTORY_SEARCH_LOADING,
        payload: {
            isLoading
        }
    }
}

export function inventorySearchResults(results) {
    return {
        type: INVENTORY_SEARCH_RESULTS,
        payload: {
            isLoading: false,
            results: results
        }
    }
}

export function inventorySearchError(error) {
    return {
        type: INVENTORY_SEARCH_ERROR,
        payload: {
            isLoading: false
        },
        error
    }
}

export function inventorySearch(searchTerms) {
    // Return a function that accepts `dispatch` so we can dispatch later.
    // Thunk middleware knows how to turn thunk async actions into actions.
    return (dispatch) => {

        dispatch(inventorySearchLoading(true));

        let apiUrl;
        let merchantId = 1; // TODO: extract from user profille

        if (process.env.__CLIENT__) {

            apiUrl = '/api/inventory/search';

        } else {

            let domainUrl = process.env.INVENTORY_API_URL || 'http://my.development.url'
            let pathtree = '/api/inventory/query/merchant/' + merchantId;

            apiUrl = domainUrl + pathtree;

        }

        let options = {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify({
                id: searchTerms.id,
                merchantId,
                sku: searchTerms.sku,
                location: null,
                zone: null,
                isPickable: searchTerms.activeOnly
            })
        };
        // add merchant ID header value to send to API from FE Server
        if (!process.env.__CLIENT__) {
            options.headers['MerchantId'] = merchantId;
        }

        return fetch(apiUrl + pathtree, options).then((response) => {
            if (!response.ok) {
                throw Error(response.statusText);
            }

            dispatch(inventorySearchLoading(false));

        })
            .then((response) => response.json())
            .then((items) => dispatch(inventorySearchResults(items)))
            .catch(() => dispatch(inventorySearchError(error)));

    };
}

inventoryreducer.js

import { INVENTORY_SEARCH_LOADING, INVENTORY_SEARCH_RESULTS, INVENTORY_SEARCH_ERROR } from '../actions/inventoryActions';

const initialState = {
    isLoading: false,
    search: {
        terms: {
            id: '',
            sku: '',
            styleId: '',
            name: '',
            vendor: '',
            location: '',
            zone: '',
            isPickable: true,
            rows: 50
        }
    },
    results: [],
    selectedItemId: '',
    selectedItem: {}
};

function inventoryReducer (state = initialState, action) {
    const { type, error, payload } = action;

    switch (type) {
        case INVENTORY_SEARCH_LOADING:
            return payload.isLoading;
        case INVENTORY_SEARCH_RESULTS:
            return payload.results;
        case INVENTORY_SEARCH_ERROR:
            return error;
        default:
            return state;
    }
};

export default inventoryReducer;

InventorySearchForm.js

/**
 * Created by raumer on 1/24/17.
 */
import React, { Component } from 'react';
import { connect } from 'react-redux';
import { inventorySearch } from './../../../../redux/actions/inventory'

class AdjustmentsForm extends Component {
    constructor(props) {
        super(props);
    }
    handleSubmit() {
        let searchTerms = {};
        // attach form elements to search terms
        this.props.performSearch(searchTerms)
    }

    render() {
        if (this.props.isLoading) {
            return <p4>Loading...</p4>
        }
        return (
            <div>
                <form>
                    <fieldset>
                        <legend>search criteria</legend>
                        <div>
                            <label htmlFor="">Sku</label>
                            <input type="text"/>
                        </div>
                        <div>
                            <label htmlFor="">Style ID</label>
                            <input type="text"/>
                        </div>
                        <div>
                            <label htmlFor="">Product Name</label>
                            <input type="text"/>
                        </div>
                        <div>
                            <label htmlFor="">Vendor Brand</label>
                            <input type="text"/>
                        </div>
                        <div>
                            <label htmlFor="">active only?</label>
                            <input type="checkbox"/>
                        </div>
                        <div>
                            <label htmlFor="">Number of Rows</label>
                            <input type="dropdown"/>
                        </div>
                    </fieldset>
                    <button type="submit" onClick={this.handleSubmit()}>Search</button>
                </form>
            </div>
        )
    }
}

const mapStateToProps = (state) => {

    return {
        searchTerms: state.inventory.search.terms,
        isLoading: state.inventory.isLoading
    }
};

const  mapDispatchToProps = (dispatch) => {
    return {
        performSearch: (searchTerms) => dispatch(inventorySearch(searchTerms))
    }
};

export default connect(mapStateToProps, mapDispatchToProps)(AdjustmentsForm);

reducers.js

import { routerReducer } from 'react-router-redux';
import { combineReducers } from 'redux';
import authReducer from './auth';
import errorMessage from './pageErrorMessage';
import inventoryReducer from './inventoryreducer';

const rootReducer = combineReducers({
    pageError: errorMessage,
    auth: authReducer,
    inventory: inventoryReducer,
    router: routerReducer,
});

export default rootReducer;

Given this App setup, why do I get this error?

"Error: Actions must be plain objects. Use custom middleware for async actions."

at dispatch (/Users/raumer/PATH_TO_PROJECT/node_modules/redux/lib/createStore.js:166:13)
      at Object.performSearch (/Users/raumer/PATH_TO_PROJECT/app/shared/screens/inventory/adjustments/adjustmentsform.js:69:41)
      at AdjustmentsForm.handleSubmit (/Users/raumer/PATH_TO_PROJECT/app/shared/screens/inventory/adjustments/adjustmentsform.js:15:20)
      at AdjustmentsForm.render (/Users/raumer/PATH_TO_PROJECT/app/shared/screens/inventory/adjustments/adjustmentsform.js:52:57)
      at /Users/raumer/PATH_TO_PROJECT/node_modules/react-dom/lib/ReactCompositeComponent.js:796:21