React-dnd: Error no detectado: el elemento debe ser un objeto

Hola. Intenté encontrar alguna mención de este error, pero desafortunadamente no había nada aquí o en la web.

Comencé un proyecto y tengo una configuración bastante simple y cuando intento arrastrar un elemento, tengo este error:
http://prntscr.com/jpfale

Este es mi componente de arrastre:

import React from 'react';
import PropTypes from 'prop-types';
import styled from 'styled-components';
import { DragSource } from 'react-dnd';

const Container = styled.span`
  margin: 10px;
  font-size: 4rem;
  text-transform: uppercase;
  cursor: pointer;
  transition: 0.4s;

  &.is-clicked {
    color: ${({ theme }) => theme.colors.purple};
  }

  &.is-hidden {
    opacity: 0;
  }
`;

const spec = {
  beginDrag(props) {
    console.log('test');
    return {};
  },
};

const WordItem = ({
  children,
  item,
  isHidden,
  handleClick,
  connectDragSource,
}) => {
  return connectDragSource(
    <span>
      <Container
        className={item.clicked ? 'is-clicked' : isHidden}
        onClick={handleClick}
      >
        {children}
      </Container>
    </span>,
  );
};

WordItem.propTypes = {
  children: PropTypes.string.isRequired,
  item: PropTypes.objectOf(PropTypes.any).isRequired,
  isHidden: PropTypes.string.isRequired,
  handleClick: PropTypes.func.isRequired,
};

export default DragSource('word', spec, (connect, monitor) => ({
  connectDragSource: connect.dragSource(),
  isDragging: monitor.isDragging(),
  dropResult: monitor.getDropResult(),
  canDrag: monitor.canDrag(),
}))(WordItem);

Contenedor conectado con el contexto:

export default DragDropContext(HTML5Backend)(App);

Ya hice un proyecto con react-dnd, pero esta es la primera vez que veo este problema. Aquí está mi package.json si este problema podría estar relacionado con la compatibilidad de bibliotecas:

{
  "name": "",
  "version": "1.0.0",
  "description": "npm i -g prettier",
  "main": "index.js",
  "scripts": {
    "clean": "rimraf dist",
    "dev": "cross-env NODE_ENV=development node devServer.js",
    "build": "npm run clean && cross-env NODE_ENV=production webpack --config=webpack.build.config.js -p && node productionServer.js",
    "flow": "flow"
  },
  "author": "",
  "license": "ISC",
  "browserslist": [
    "last 5 versions",
    "not ie < 11"
  ],
  "prettier": {
    "singleQuote": true,
    "trailingComma": "all"
  },
  "dependencies": {
    "axios": "^0.18.0",
    "babel-plugin-styled-components": "^1.5.1",
    "es6-promise": "^4.2.4",
    "prop-types": "^15.6.1",
    "rc-slider": "^8.6.1",
    "react": "^16.3.2",
    "react-dnd": "^3.0.2",
    "react-dnd-html5-backend": "^3.0.2",
    "react-dom": "^16.3.2",
    "react-motion": "^0.5.2",
    "react-redux": "^5.0.7",
    "react-router-dom": "^4.2.2",
    "redux": "^4.0.0",
    "redux-thunk": "^2.2.0",
    "styled-components": "^3.2.6",
    "svg-inline-loader": "^0.8.0",
    "xml2js": "^0.4.19"
  },
  "devDependencies": {
    "autoprefixer": "^8.3.0",
    "babel-core": "^6.26.0",
    "babel-eslint": "^8.2.3",
    "babel-loader": "^7.1.4",
    "babel-plugin-transform-class-properties": "^6.24.1",
    "babel-plugin-transform-object-rest-spread": "^6.26.0",
    "babel-plugin-transform-runtime": "^6.23.0",
    "babel-preset-env": "^1.6.1",
    "babel-preset-flow": "^6.23.0",
    "babel-preset-react": "^6.24.1",
    "browserslist": "^3.2.4",
    "compression-webpack-plugin": "^1.1.11",
    "cross-env": "^5.1.4",
    "css-loader": "^0.28.11",
    "eslint": "^4.19.1",
    "eslint-config-airbnb": "^16.1.0",
    "eslint-config-prettier": "^2.9.0",
    "eslint-plugin-import": "^2.11.0",
    "eslint-plugin-jsx-a11y": "^6.0.3",
    "eslint-plugin-prettier": "^2.6.0",
    "eslint-plugin-react": "^7.7.0",
    "express": "^4.16.3",
    "extract-text-webpack-plugin": "^4.0.0-beta.0",
    "file-loader": "^1.1.11",
    "flow-bin": "^0.70.0",
    "html-webpack-plugin": "^3.2.0",
    "image-webpack-loader": "^4.2.0",
    "node-sass": "^4.8.3",
    "postcss-loader": "^2.1.4",
    "prettier-eslint": "^8.8.1",
    "react-hot-loader": "^4.0.0",
    "rimraf": "^2.6.2",
    "sass-loader": "^7.0.1",
    "style-loader": "^0.20.3",
    "url-loader": "^1.0.1",
    "webpack": "^4.6.0",
    "webpack-cli": "^2.1.2",
    "webpack-dev-middleware": "^3.1.2",
    "webpack-hot-middleware": "^2.22.1"
  }
}

Ayude a resolver este problema.