diff --git a/lib/AvaTaxClient.js b/lib/AvaTaxClient.js
index 6210d17f..966e9cbe 100644
--- a/lib/AvaTaxClient.js
+++ b/lib/AvaTaxClient.js
@@ -14,7 +14,7 @@
  * @link       https://github.com/avadev/AvaTax-REST-V2-JS-SDK
  */
 
-import fetch from 'isomorphic-fetch';
+import fetch from 'node-fetch';
 import { createBasicAuthHeader } from './utils/basic_auth';
 import { withTimeout } from './utils/withTimeout';
 
@@ -30,14 +30,14 @@ export default class AvaTaxClient {
    * @param number timeout      Specify the timeout for AvaTax requests; default value 20 minutes.
    */
   constructor({ appName, appVersion, machineName, environment, timeout = 1200000 }) {
-    this.appNM=appName;
-	this.appVer=appVersion;
-	this.machineNM=machineName
+    this.appNM = appName;
+	  this.appVer = appVersion;
+	  this.machineNM = machineName;
     this.baseUrl = 'https://rest.avatax.com';
     if (environment == 'sandbox') {
       this.baseUrl = 'https://sandbox-rest.avatax.com';
     } else if (
-      typeof environment !== "undefined" &&
+      typeof environment !== 'undefined' &&
       (environment.substring(0, 8) == 'https://' ||
       environment.substring(0, 7) == 'http://')
     ) {
@@ -74,45 +74,38 @@ export default class AvaTaxClient {
    * @param   string  verb       The HTTP verb being used in this request
    * @param   string  payload    The request body, if this is being sent to a POST/PUT API call
    */
-  restCall({ url, verb, payload, clientId="",mapHeader = new Map() }) {
-    const reqHeaders = new Headers({
+  restCall({ url, verb, payload, clientId = '', mapHeader = new Map() }) {
+    const reqHeaders = {
         Accept: 'application/json',
         'Content-Type': 'application/json',
         Authorization: this.auth,
         'X-Avalara-Client': clientId
-      });    
+    };    
     for (let [key, value] of mapHeader) {
-      reqHeaders.append(key,value);
+      reqHeaders[key] = value;
     }
     return withTimeout(this.timeout, fetch(url, {
       method: verb,
       headers: reqHeaders,
-      body: JSON.stringify(payload)
+      body: payload == null ? null : JSON.stringify(payload)
     })).then(res => {
-	  var contentType = res.headers._headers['content-type'];
-	  var contentLength = res.headers._headers['content-length'];
-    if (typeof contentLength !== "undefined" && contentLength != null)
-    {
-      contentLength = res.headers._headers['content-length'][0];
-    }
-
-    if (contentType[0] === 'application/vnd.ms-excel' || contentType[0] === 'text/csv') {
+      const contentType = res.headers.get('content-type');
+      const contentLength = res.headers.get('content-length');
+      if (contentType === 'application/vnd.ms-excel' || contentType === 'text/csv') {
         return res;
       }
 
-    if (res.headers._headers['content-type'].includes('application/json'))
-    {
-      if (typeof contentLength !== "undefined" && contentLength != null && contentLength == 0 && parseInt(res.status/100)==2 ){
-        return null;
+      if (contentType && contentType.includes('application/json')) {
+        if (contentLength == 0 && parseInt(res.status/100)==2 ){
+          return null;
+        }
       }
-    }
       return res.json().catch((error) => {
-        let ex = new Error("The server returned the response is in an unexpected format");
+        let ex = new Error('The server returned the response is in an unexpected format');
         ex.code = 'FormatException';
         ex.details = error;
         throw ex;
-      });
-    }).then(json => {
+      }).then(json => {
         // handle error
         if (json &&  json.error) {
           let ex = new Error(json.error.message);
@@ -123,7 +116,8 @@ export default class AvaTaxClient {
         } else {
           return json;
         }
-      })      
+      });      
+    });
   }
 
   /**