diff --git a/src/html/template/template.go b/src/html/template/template.go
index b313a6b104..e1f179abc2 100644
--- a/src/html/template/template.go
+++ b/src/html/template/template.go
@@ -325,6 +325,7 @@ func (t *Template) Name() string {
 type FuncMap map[string]interface{}
 
 // Funcs adds the elements of the argument map to the template's function map.
+// It must be called before the template is parsed.
 // It panics if a value in the map is not a function with appropriate return
 // type. However, it is legal to overwrite elements of the map. The return
 // value is the template, so calls can be chained.
diff --git a/src/text/template/template.go b/src/text/template/template.go
index b6fceb1795..ed1ef3cf8d 100644
--- a/src/text/template/template.go
+++ b/src/text/template/template.go
@@ -159,6 +159,7 @@ func (t *Template) Delims(left, right string) *Template {
 }
 
 // Funcs adds the elements of the argument map to the template's function map.
+// It must be called before the template is parsed.
 // It panics if a value in the map is not a function with appropriate return
 // type or if the name cannot be used syntactically as a function in a template.
 // It is legal to overwrite elements of the map. The return value is the template,