diff --git a/src/plugins/projectexplorer/project.cpp b/src/plugins/projectexplorer/project.cpp
index f91f2fa1a0e..a17584ea40d 100644
--- a/src/plugins/projectexplorer/project.cpp
+++ b/src/plugins/projectexplorer/project.cpp
@@ -505,6 +505,7 @@ void Project::setDisplayName(const QString &name)
 
 void Project::setId(Core::Id id)
 {
+    QTC_ASSERT(!d->m_id.isValid(), return); // Id may not change ever!
     d->m_id = id;
 }