composer.json

@@ -22,6 +22,7 @@
     },
     "require-dev": {
         "ext-filter": "*",
+        "ext-pcre": "*",
         "bamarni/composer-bin-plugin": "^1.3",
         "phpunit/phpunit": "^4.8.35 || ^5.0 || ^6.0 || ^7.0"
     },
@@ -36,7 +37,8 @@
         }
     },
     "suggest": {
-        "ext-filter": "Required to use the boolean validator."
+        "ext-filter": "Required to use the boolean validator.",
+        "ext-pcre": "Required to use most of the library."
     },
     "extra": {
         "branch-alias": {

src/Validator.php

@@ -128,6 +128,10 @@ public function allowedValues(array $choices)
     {
         return $this->assertCallback(
             function ($value) use ($choices) {
+                if ($value === null) {
+                    return true;
+                }
+
                 return in_array($value, $choices, true);
             },
             sprintf('is not one of [%s]', implode(', ', $choices))

tests/Dotenv/ValidatorTest.php

@@ -41,6 +41,14 @@ public function testDotenvAllowedValuesIfPresent()
         $this->assertTrue(true);
     }
 
+    public function testDotenvAllowedValuesIfNotPresent()
+    {
+        $dotenv = Dotenv::createImmutable($this->fixturesFolder);
+        $dotenv->load();
+        $dotenv->ifPresent('FOOQWERTYOOOOOO')->allowedValues(['bar', 'baz']);
+        $this->assertTrue(true);
+    }
+
     /**
      * @expectedException \Dotenv\Exception\ValidationException
      * @expectedExceptionMessage One or more environment variables failed assertions: FOO is not one of [buzz, buz].