diff --git a/ext/pimple/pimple.c b/ext/pimple/pimple.c
index b2dd6176..c80499b3 100644
--- a/ext/pimple/pimple.c
+++ b/ext/pimple/pimple.c
@@ -715,10 +715,16 @@ PHP_METHOD(Pimple, extend)
 				RETURN_NULL();
 			}
 
-			if (value->type != PIMPLE_IS_SERVICE || zend_hash_index_exists(&pobj->protected, value->handle_num)) {
+			if (value->type != PIMPLE_IS_SERVICE) {
 				pimple_throw_exception_string(pimple_ce_InvalidServiceIdentifierException, Z_STRVAL_P(offset), Z_STRLEN_P(offset) TSRMLS_CC);
 				RETURN_NULL();
 			}
+			if (zend_hash_index_exists(&pobj->protected, value->handle_num)) {
+				int er = EG(error_reporting);
+				EG(error_reporting) = 0;
+				php_error(E_DEPRECATED, "How Pimple behaves when extending protected closures will be fixed in Pimple 4. Are you sure \"%s\" should be protected?", Z_STRVAL_P(offset));
+				EG(error_reporting) = er;
+			}
 		break;
 		case IS_DOUBLE:
 		case IS_BOOL:
@@ -733,11 +739,17 @@ PHP_METHOD(Pimple, extend)
 				pimple_throw_exception_string(pimple_ce_UnknownIdentifierException, Z_STRVAL_P(offset), Z_STRLEN_P(offset) TSRMLS_CC);
 				RETURN_NULL();
 			}
-			if (value->type != PIMPLE_IS_SERVICE || zend_hash_index_exists(&pobj->protected, value->handle_num)) {
+			if (value->type != PIMPLE_IS_SERVICE) {
 				convert_to_string(offset);
 				pimple_throw_exception_string(pimple_ce_InvalidServiceIdentifierException, Z_STRVAL_P(offset), Z_STRLEN_P(offset) TSRMLS_CC);
 				RETURN_NULL();
 			}
+			if (zend_hash_index_exists(&pobj->protected, value->handle_num)) {
+				int er = EG(error_reporting);
+				EG(error_reporting) = 0;
+				php_error(E_DEPRECATED, "How Pimple behaves when extending protected closures will be fixed in Pimple 4. Are you sure \"%ld\" should be protected?", index);
+				EG(error_reporting) = er;
+			}
 		break;
 		case IS_NULL:
 		default:
diff --git a/src/Pimple/Container.php b/src/Pimple/Container.php
index 73a4ebcd..e761964b 100644
--- a/src/Pimple/Container.php
+++ b/src/Pimple/Container.php
@@ -241,10 +241,14 @@ public function extend($id, $callable)
             throw new FrozenServiceException($id);
         }
 
-        if (!is_object($this->values[$id]) || !method_exists($this->values[$id], '__invoke') || isset($this->protected[$this->values[$id]])) {
+        if (!is_object($this->values[$id]) || !method_exists($this->values[$id], '__invoke')) {
             throw new InvalidServiceIdentifierException($id);
         }
 
+        if (isset($this->protected[$this->values[$id]])) {
+            @trigger_error(sprintf('How Pimple behaves when extending protected closures will be fixed in Pimple 4. Are you sure "%s" should be protected?', $id), E_USER_DEPRECATED);
+        }
+
         if (!is_object($callable) || !method_exists($callable, '__invoke')) {
             throw new ExpectedInvokableException('Extension service definition is not a Closure or invokable object.');
         }
diff --git a/src/Pimple/Tests/PimpleTest.php b/src/Pimple/Tests/PimpleTest.php
index 09a640f7..acb66e00 100644
--- a/src/Pimple/Tests/PimpleTest.php
+++ b/src/Pimple/Tests/PimpleTest.php
@@ -392,17 +392,21 @@ public function testLegacyExtendFailsForKeysNotContainingServiceDefinitions($ser
     }
 
     /**
-     * @expectedException \Pimple\Exception\InvalidServiceIdentifierException
-     * @expectedExceptionMessage Identifier "foo" does not contain an object definition.
+     * @group legacy
+     * @expectedDeprecation How Pimple behaves when extending protected closures will be fixed in Pimple 4. Are you sure "foo" should be protected?
      */
-    public function testExtendFailsIfEntryIsProtected()
+    public function testExtendingProtectedClosureDeprecation()
     {
         $pimple = new Container();
         $pimple['foo'] = $pimple->protect(function () {
             return 'bar';
         });
 
-        $pimple->extend('foo', function () {});
+        $pimple->extend('foo', function ($value) {
+            return $value.'-baz';
+        });
+
+        $this->assertSame('bar-baz', $pimple['foo']);
     }
 
     /**