diff --git a/lib/symbols b/lib/symbols
index 64427b3aaa..f6611142a3 100644
--- a/lib/symbols
+++ b/lib/symbols
@@ -18,6 +18,9 @@
 # or
 # \def\macroname{definition} extra xmlname requires
 #
+# ("" for empty strings is not supported. One would have to use x for fields
+# that are not set.)
+#
 # If you put any symbol with a requirement inside an iffont clause, you
 # must put an alternative definition with the same requirement in the
 # corresponding else clause. Otherwise the LaTeX output would depend on the