diff --git a/cfg.mk b/cfg.mk
index 74fe5ed4c9..dcf73c4185 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -133,13 +133,13 @@ sc_prohibit_ctype_h:
 	halt="don't use ctype.h; instead, use c-ctype.h"		\
 	 $(_sc_search_regexp)
 
-# Ensure that no C source file uses TABs for indentation.
-# Also match *.h.in files, to get libvirt.h.in.
-# Exclude files in gnulib, since they're imported.
+# Ensure that no C source file or rng schema uses TABs for
+# indentation.  Also match *.h.in files, to get libvirt.h.in.  Exclude
+# files in gnulib, since they're imported.
 sc_TAB_in_indentation:
 	@prohibit='^ *	'						\
 	in_vc_files='\.(rng|[ch](\.in)?)$$'				\
-	halt='use spaces, not TAB, for indentation in C sources'	\
+	halt='use spaces, not TAB, for indentation in C sources and RNG schemas' \
 	 $(_sc_search_regexp)
 
 ctype_re = isalnum|isalpha|isascii|isblank|iscntrl|isdigit|isgraph|islower\