diff --git a/runtime/doc/map.txt b/runtime/doc/map.txt
index 0ad43d4..79c46f7 100644
--- a/runtime/doc/map.txt
+++ b/runtime/doc/map.txt
@@ -1167,6 +1167,10 @@
 the script it was defined in.  This makes it possible that the command calls a
 local function or uses a local mapping.
 
+In case the value is used in a context where <SID> cannot be correctly
+expanded, use the expand() function: >
+	let &includexpr = expand('<SID>') .. 'My_includeexpr()'
+
 Otherwise, using "<SID>" outside of a script context is an error.
 
 If you need to get the script number to use in a complicated script, you can