diff --git a/qtcreatorplugin.pri b/qtcreatorplugin.pri
index 6579467a0d7..190dbed1c3b 100644
--- a/qtcreatorplugin.pri
+++ b/qtcreatorplugin.pri
@@ -1,5 +1,5 @@
-IDE_SOURCE_TREE=$$(QTC_SOURCE)
-IDE_BUILD_TREE=$$(QTC_BUILD)
+isEmpty(IDE_SOURCE_TREE): IDE_SOURCE_TREE=$$(QTC_SOURCE)
+isEmpty(IDE_BUILD_TREE): IDE_BUILD_TREE=$$(QTC_BUILD)
 
 isEmpty(IDE_SOURCE_TREE):error(Set QTC_SOURCE environment variable)
 isEmpty(IDE_BUILD_TREE):error(Set QTC_BUILD environment variable)