diff --git a/src/ContainerBuilder.php b/src/ContainerBuilder.php
index 92d72bf4..7a7285c7 100644
--- a/src/ContainerBuilder.php
+++ b/src/ContainerBuilder.php
@@ -247,7 +247,7 @@ public function writeProxiesToFile(bool $writeToFile, string $proxyDirectory = n
                 'The proxy directory must be specified if you want to write proxies on disk'
             );
         }
-        $this->proxyDirectory = $proxyDirectory;
+        $this->proxyDirectory = $writeToFile ? $proxyDirectory : null;
 
         return $this;
     }
diff --git a/tests/UnitTest/ContainerBuilderTest.php b/tests/UnitTest/ContainerBuilderTest.php
index 1a863cb7..c88062e7 100644
--- a/tests/UnitTest/ContainerBuilderTest.php
+++ b/tests/UnitTest/ContainerBuilderTest.php
@@ -21,18 +21,19 @@
 class ContainerBuilderTest extends TestCase
 {
     use EasyMock;
+	
+	private static function getProperty(object $object, string $propertyName)
+	{
+		return (function (string $propertyName) {
+			return $this->$propertyName;
+		})->bindTo($object, $object)($propertyName);
+	}
 
     /**
      * @test
      */
     public function should_configure_for_development_by_default()
     {
-        $getProperty = function (object $object, string $propertyName) {
-            return (function (string $propertyName) {
-                return $this->$propertyName;
-            })->bindTo($object, $object)($propertyName);
-        };
-
         // Make the ContainerBuilder use our fake class to catch constructor parameters
         $builder = new ContainerBuilder(FakeContainer::class);
         /** @var FakeContainer $container */
@@ -41,7 +42,7 @@ public function should_configure_for_development_by_default()
         // Not compiled
         $this->assertNotInstanceOf(CompiledContainer::class, $container);
         // Proxies evaluated in memory
-        $this->assertNull($getProperty($container->proxyFactory, 'proxyDirectory'));
+        $this->assertNull(self::getProperty($container->proxyFactory, 'proxyDirectory'));
     }
 
     /**
@@ -248,4 +249,28 @@ public function should_throw_if_modified_after_building_a_container()
 
         $builder->addDefinitions([]);
     }
+
+    /**
+     * @test
+     */
+    public function should_create_proxies()
+    {
+        $builder = new ContainerBuilder(FakeContainer::class);
+        $builder->writeProxiesToFile(true, 'somedir');
+        $container = $builder->build();
+
+        $this->assertSame('somedir', self::getProperty($container->proxyFactory, 'proxyDirectory'));
+    }
+
+    /**
+     * @test
+     */
+    public function should_not_create_proxies()
+    {
+        $builder = new ContainerBuilder(FakeContainer::class);
+        $builder->writeProxiesToFile(false, 'somedir');
+        $container = $builder->build();
+
+        $this->assertNull(self::getProperty($container->proxyFactory, 'proxyDirectory'));
+    }
 }