diff --git a/v1/allyourbase.jam b/v1/allyourbase.jam
index aff37d94f..982613f1e 100644
--- a/v1/allyourbase.jam
+++ b/v1/allyourbase.jam
@@ -526,6 +526,12 @@ DEPENDS all shell files lib dll exe obj test : first ;
 NOTFILE all first shell files lib dll exe obj dirs clean uninstall test nothing ;
 ALWAYS clean uninstall ;
 
+#
+# Prevent external variables from interfeering with some esnetial intenal
+# variables.
+#
+TOP = ;
+
 #
 # Rules
 #