diff --git a/scripts/Mmake.vars.in b/scripts/Mmake.vars.in
index ca73c7078..74694f7b5 100644
--- a/scripts/Mmake.vars.in
+++ b/scripts/Mmake.vars.in
@@ -50,7 +50,13 @@ endif
 # occurrence of $(patsubst ...), since GNU Make does not respect quotes.
 UNIX_MERCURY_EXTRA_INT_DIRS = $(foreach dir, $(MERCURY_EXTRA_INT_DIRS),\
 				$(shell @CYGPATHU@ "$(dir)"))
-UNIX_MERC_INT_DIR = $(shell @CYGPATHU@ "$(MERC_INT_DIR)")
+
+ifdef MERC_INT_DIR
+    UNIX_MERC_INT_DIR = $(shell @CYGPATHU@ "$(MERC_INT_DIR)")
+else
+    UNIX_MERC_INT_DIR =
+endif
+   
 UNIX_MERCURY_EXTRA_INIT_DIRS = $(foreach dir, $(MERCURY_EXTRA_INIT_DIRS),\
 				$(shell @CYGPATHU@ "$(dir)"))