diff --git a/addons/web/static/src/js/core/utils.js b/addons/web/static/src/js/core/utils.js
index d6165d0a841..1c2dbd35ff9 100644
--- a/addons/web/static/src/js/core/utils.js
+++ b/addons/web/static/src/js/core/utils.js
@@ -227,7 +227,22 @@ function _getExtractorFrom(criterion) {
     }
 }
 
+class AlreadyDefinedPatchError extends Error {
+    constructor() {
+        super(...arguments);
+        this.name = 'AlreadyDefinedPatchError';
+    }
+}
+class UnknownPatchError extends Error {
+    constructor() {
+        super(...arguments);
+        this.name = 'UnknownPatchError';
+    }
+}
+
 var utils = {
+    AlreadyDefinedPatchError,
+    UnknownPatchError,
 
     /**
      * Throws an error if the given condition is not true
@@ -627,7 +642,7 @@ var utils = {
         }
         const objDesc = patchMap.get(obj);
         if (objDesc.patches.some(p => p.name === patchName)) {
-            throw new Error(`Class ${obj.name} already has a patch ${patchName}`);
+            throw new AlreadyDefinedPatchError(`Patch ${patchName} is already defined`);
         }
         objDesc.patches.push({
             name: patchName,
@@ -901,8 +916,8 @@ var utils = {
      */
     unpatch: function (obj, patchName) {
         const objDesc = patchMap.get(obj);
-        if (!objDesc.patches.some(p => p.name === patchName)) {
-            throw new Error(`Class ${obj.name} does not have any patch ${patchName}`);
+        if (!objDesc || !objDesc.patches.some(p => p.name === patchName)) {
+            throw new UnknownPatchError(`Could not find patch ${patchName}`);
         }
         patchMap.delete(obj);
 
diff --git a/addons/web/static/tests/core/patch_tests.js b/addons/web/static/tests/core/patch_tests.js
index 4668bdf217c..ee6c579cbc5 100644
--- a/addons/web/static/tests/core/patch_tests.js
+++ b/addons/web/static/tests/core/patch_tests.js
@@ -1,7 +1,7 @@
 odoo.define("web.patch_tests", function (require) {
 "use strict";
 
-const { patch, unpatch } = require('web.utils');
+const { AlreadyDefinedPatchError, patch, unpatch, UnknownPatchError } = require('web.utils');
 
 function makeBaseClass(assert, assertInSetup) {
     class BaseClass {
@@ -122,7 +122,7 @@ QUnit.module('core', {}, function () {
             // keys should be unique
             assert.throws(() => {
                 patch(A.prototype, 'patch');
-            });
+            }, AlreadyDefinedPatchError);
         });
 
         QUnit.test('unpatch', async function (assert) {
@@ -278,6 +278,15 @@ QUnit.module('core', {}, function () {
             ]);
         });
 
+        QUnit.test('unpatch a class that has not been patched', async function (assert) {
+            assert.expect(1);
+
+            const A = class {};
+            assert.throws(() => {
+                unpatch(A.prototype, 'patch');
+            }, UnknownPatchError);
+        });
+
         QUnit.test('unpatch twice the same patch name', async function (assert) {
             assert.expect(1);
 
@@ -287,7 +296,7 @@ QUnit.module('core', {}, function () {
             unpatch(A.prototype, 'patch');
             assert.throws(() => {
                 unpatch(A.prototype, 'patch');
-            });
+            }, UnknownPatchError);
         });
 
         QUnit.test('unpatch and re-patch', async function (assert) {