2 files changed, 4 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index 69234285..414ec679 100644
--- a/NEWS
+++ b/NEWS
@@ -6,7 +6,8 @@ This file documents the major additions and syntax changes between releases.
         Fix check_ircd binding to wrong interface (#668778)
         Add proxy-authorization option to check_http (Marcel Kuiper - #1323230, Bryan Irvine - #2863925)
         WARNINGS
-        Updated developer documentation to say that performance labels should not have an equals sign in the label
+        Updated developer documentation to say that performance labels should not have an equals sign or
+        single quote in the label
 
 1.4.14 16th September 2009
         check_http has options to specify the HTTP method (#2155152)
diff --git a/doc/developer-guidelines.sgml b/doc/developer-guidelines.sgml
index 3567841a..d6c7c075 100644
--- a/doc/developer-guidelines.sgml
+++ b/doc/developer-guidelines.sgml
@@ -333,10 +333,10 @@
                 <orderedlist>
                 <listitem><para>space separated list of label/value pairs</para>
                         </listitem>
-                <listitem><para>label can contain any characters</para>
+                <listitem><para>label can contain any characters except the equals sign or single quote (')</para>
                         </listitem>
                 <listitem><para>the single quotes for the label are optional. Required if 
-                        spaces, = or ' are in the label</para>
+                        spaces are in the label</para>
                         </listitem>
                 <listitem><para>label length is arbitrary, but ideally the first 19 characters
                         are unique (due to a limitation in RRD). Be aware of a limitation in the