diff --git a/mkspecs/features/qt_tool.prf b/mkspecs/features/qt_tool.prf
index 3f0301a292..839c3d6b88 100644
--- a/mkspecs/features/qt_tool.prf
+++ b/mkspecs/features/qt_tool.prf
@@ -64,3 +64,5 @@ DEFINES *= QT_USE_QSTRINGBUILDER
         cache(QT_TOOL.$${MODULE}.$$var, transient)
 
 }
+# The variable is re-used by qtPrepareTool(), and we really don't want that.
+unset(QT_TOOL_ENV)