diff --git a/lix/libexpr/builtins/substring.md b/lix/libexpr/builtins/substring.md
index a10d6b4cf..09ffee736 100644
--- a/lix/libexpr/builtins/substring.md
+++ b/lix/libexpr/builtins/substring.md
@@ -5,12 +5,18 @@ args: [start, len, s]
 Return the substring of *s* from character position *start*
 (zero-based) up to but not including *start + len*. If *start* is
 greater than the length of the string, an empty string is returned,
-and if *start + len* lies beyond the end of the string, only the
-substring up to the end of the string is returned. *start* must be
-non-negative. For example,
+and if *start + len* lies beyond the end of the string or *len*
+is negative, only the substring up to the end of the string is
+returned. *start* must be non-negative. For example,
 
 ```nix
 builtins.substring 0 3 "nixos"
 ```
 
-evaluates to `"nix"`.
+evaluates to `"nix"`, and
+
+```nix
+builtins.substring 3 (-1) "nixos"
+```
+
+evaluates to `"os"`.