diff --git a/harbour/makefile64.nt b/harbour/makefile64.nt
index 0e0798bbce..685fb764b3 100644
--- a/harbour/makefile64.nt
+++ b/harbour/makefile64.nt
@@ -132,7 +132,7 @@ CFLAGS = $(CFLAGS) -DSIMPLEX
 #
 
 AS             = ML
-CFLAGS         = -I$(INCLUDE_DIR) -TP -W3 -nologo $(C_USR) $(CFLAGS)
+CFLAGS         = -I$(INCLUDE_DIR) -TP -W3 -D__64__ -nologo $(C_USR) $(CFLAGS)
 CLIBFLAGS      = -c $(CFLAGS) $(CLIBFLAGS)
 CLIBFLAGSDEBUG = -Zi $(CLIBFLAGS)
 HARBOURFLAGS   = -i$(INCLUDE_DIR) -n -q0 -w -es2 -gc0 $(PRG_USR) $(HARBOURFLAGS)