diff --git a/core/modules/user/src/PermissionHandler.php b/core/modules/user/src/PermissionHandler.php
index 715b739..a574cc3 100644
--- a/core/modules/user/src/PermissionHandler.php
+++ b/core/modules/user/src/PermissionHandler.php
@@ -132,7 +132,12 @@ protected function buildPermissionsYaml($module_name = NULL) {
 
     if ($module_name) {
       // Just discover the single module.
-      $discovery = new YamlDiscovery('permissions', [$module_name => $this->moduleHandler->getModuleDirectories()[$module_name]]);
+      $directories = $this->moduleHandler->getModuleDirectories();
+      $module_directories = [];
+      if (!empty($directories[$module_name])) {
+        $module_directories = [$module_name => $directories[$module_name]];
+      }
+      $discovery = new YamlDiscovery('permissions', $module_directories);
     }
     else {
       // Use the default discovery - which handles all enabled modules.
diff --git a/core/modules/user/tests/src/Unit/PermissionHandlerTest.php b/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
index 1180846..cd7fbcc 100644
--- a/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
+++ b/core/modules/user/tests/src/Unit/PermissionHandlerTest.php
@@ -92,7 +92,7 @@ public function testBuildPermissionsYaml() {
     vfsStreamWrapper::setRoot($root);
 
     $this->moduleHandler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
-    $this->moduleHandler->expects($this->once())
+    $this->moduleHandler->expects($this->any())
       ->method('getModuleDirectories')
       ->willReturn(array(
         'module_a' => vfsStream::url('modules/module_a'),