res.json is not a function ( I connect reactjs to API)

dataset.list.js

import React, { Component } from "react";
import Datasets from "./data";
import axios from "axios";

class App extends Component {
  render() {
    return <Datasets datasets={this.state.datasets} />;
  }

  state = {
    datasets: []
  };

  componentDidMount() {
    axios
      .get("http://localhost:3001/api/v1/datasets/findAll")
      .then(res => res.json())
      .then(data => {
        this.setState({ datasets: data });
      })
      .catch(console.log);
  }
}
export default App;

data.js

import React from "react";

const Datasets = ({ datasets }) => {
  return (
    <div>
      <center>
        <h4>Dataset list</h4>
      </center>
      {datasets.map(dataset => (
        <div class="card">
          <div class="card-body">
            <h5 class="card-title">{dataset.name}</h5>
            <h6 class="card-subtitle mb-2 text-muted">{dataset.uri}</h6>
          </div>
        </div>
      ))}
    </div>
  );
};

export default Datasets;