Value for title can not be cast from ReadablenativeMap to string

InsertDataToServer = () => {
const { pinValue1 } = this.state;
const { pinValue2 } = this.state;
const { pinValue3 } = this.state;
const { pinValue4 } = this.state;
var String_3 = pinValue1.concat("" , pinValue2);
var String_4 = String_3.concat("" ,  pinValue3);
var String_5 = String_4.concat("" ,  pinValue4);

fetch("http://www.aonde.biz/mobile/doLogin.php", {
  method: "POST",
  headers: {
    Accept: "application/json",
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "pin":212,

  })
})
  .then(response => response.json())
  .then(responseJson => {
    // Showing response message coming from server after inserting records.
    Alert.alert(responseJson);
  })
  .catch(error => {
    console.error(error);
  });

In the above code when I pass pin parameter API then show this error. Thank you please give some idea how to resolve this issue.