diff --git a/common-rules.mk b/common-rules.mk
index 4df1e33f1..7a116c827 100644
--- a/common-rules.mk
+++ b/common-rules.mk
@@ -20,6 +20,10 @@
 
 -include $(top_builddir)/localenv
 
+# Convenient way to terminate lists in Makefiles, so that we avoid
+# problems with dangling backslashes.
+NULL =
+
 # Files that should universally be removed by 'make clean'.  Note if
 # there is any case in any subdirectory where a file should not be
 # removed by 'make clean', it should not be listed here!