diff --git a/src/plugins/projectexplorer/projectexplorer.cpp b/src/plugins/projectexplorer/projectexplorer.cpp
index 73f586b9b24..8568a61be9d 100644
--- a/src/plugins/projectexplorer/projectexplorer.cpp
+++ b/src/plugins/projectexplorer/projectexplorer.cpp
@@ -1049,7 +1049,7 @@ bool ProjectExplorerPlugin::initialize(const QStringList &arguments, QString *er
 
     VariableManager::registerFileVariables(Constants::VAR_CURRENTPROJECT_PREFIX,
         tr("Current project's main file"),
-        []() {
+        []() -> QString {
             QString projectFilePath;
             if (Project *project = ProjectExplorerPlugin::currentProject())
                 if (IDocument *doc = project->document())