diff --git a/docs/ref/templates/api.txt b/docs/ref/templates/api.txt
index 2b7d35479c..03fef1da45 100644
--- a/docs/ref/templates/api.txt
+++ b/docs/ref/templates/api.txt
@@ -112,7 +112,7 @@ template::
     "My name is Dolores."
 
 Variable names must consist of any letter (A-Z), any digit (0-9), an underscore
-or a dot.
+(but they must not start with an underscore) or a dot.
 
 Dots have a special meaning in template rendering. A dot in a variable name
 signifies a **lookup**. Specifically, when the template system encounters a