diff --git a/reactos/rules.mak b/reactos/rules.mak
index f51874b828b..77ca54111cc 100644
--- a/reactos/rules.mak
+++ b/reactos/rules.mak
@@ -48,6 +48,10 @@ SEP := \$(EMPTY_VAR)
 PIPE := -pipe
 endif
 
+# Sane TOPDIR for toplevel makefile
+ifeq ($(TOPDIR),)
+TOPDIR = .
+endif
 
 # Use environment var ROS_INSTALL to override default install dir
 ifeq ($(ROS_INSTALL),)
@@ -133,4 +137,4 @@ OS2_PATH_INC=$(OS2_PATH)/include
 
 # Other systems integration
 ROOT_PATH=$(PATH_TO_TOP)/..
-REGTESTS_PATH_INC=$(PATH_TO_TOP)/regtests/shared
\ No newline at end of file
+REGTESTS_PATH_INC=$(PATH_TO_TOP)/regtests/shared