diff --git a/Container.php b/Container.php
index eddd8fd..8475e1a 100644
--- a/Container.php
+++ b/Container.php
@@ -84,6 +84,9 @@ class Container implements ContainerInterface
 		}
 		if ($this->isInterface($class)) {
 			$class = $this->_interfaces[$class];
+			if (is_null($class)) {
+				throw new Exception('Unknown class mapping ' . $class . '::class');
+			}
 		}
 		if (!isset($this->_singletons[$class])) {
 			$this->_singletons[$class] = $this->resolve($class, $constrict, $config);