diff --git a/README.md b/README.md
index 2c8a398a..565427ce 100644
--- a/README.md
+++ b/README.md
@@ -84,6 +84,8 @@ test('should mock "await import()" using esmock.p', async t => {
    <summary>changelog</summary>
    <br/>
 
+ * 1.4.0 _Nov.30.2021_
+   * throw error if esmock is called without --loader=esmock
  * 1.3.3 _Nov.28.2021_
    * update quick-start README to include phrase 'unit test'
  * 1.3.2 _Nov.27.2021_
diff --git a/package.json b/package.json
index 36ffa301..675f191f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "esmock",
-  "version": "1.3.3",
+  "version": "1.4.0",
   "license": "MIT",
   "readmeFilename": "README.md",
   "description": "provides native ESM import mocking for unit tests",
diff --git a/spec/esmock.spec.js b/spec/esmock.spec.js
index 22d2ab92..a547ad87 100644
--- a/spec/esmock.spec.js
+++ b/spec/esmock.spec.js
@@ -24,6 +24,14 @@ test('should mock a local file', async t => {
   t.is(main(), 'main string, test string');
 });
 
+test.serial('should throw error if !esmockloader', async t => {
+  esmock.esmockloader = false;
+  await t.throwsAsync(() => esmock('./to/module.js'), {
+    message : 'process must be started with --loader=esmock'
+  });
+  esmock.esmockloader = true;
+});
+
 test('should throw error if local file not found', async t => {
   await t.throwsAsync(() => esmock('./local/not/found.js', {
     './local/mainUtil.js' : {
diff --git a/src/esmock.js b/src/esmock.js
index 4ce79ff0..f065c023 100644
--- a/src/esmock.js
+++ b/src/esmock.js
@@ -10,9 +10,12 @@ import {
 
 const esmock = async (modulePath, mockDefs, globalDefs, opt = {}, err) => {
   const calleePath = (err || new Error()).stack.split('\n')[2]
-    .replace(/^.*file:\/\//, '') // rm everything before filepathfe
+    .replace(/^.*file:\/\//, '') // rm every before filepath
     .replace(/:[\d]*:[\d]*.*$/, '') // rm line and row number
-    .replace(/^.*:/, ''); // rm windows-style drive locations
+    .replace(/^.*:/, ''); // rm windows-style drive location
+
+  if (!esmock.esmockloader)
+    throw new Error('process must be started with --loader=esmock');
 
   const modulePathKey = await esmockModuleMock(
     calleePath, modulePath, mockDefs || {}, globalDefs || {}, opt);
diff --git a/src/esmockLoader.mjs b/src/esmockLoader.mjs
index 9d22f052..1019866e 100644
--- a/src/esmockLoader.mjs
+++ b/src/esmockLoader.mjs
@@ -2,6 +2,7 @@ import path from 'path';
 import url from 'url';
 
 import esmock from './esmock.js';
+esmock.esmockloader = true;
 export default esmock;
 
 // ex, file:///path/to/esmock,