diff --git a/docs/Lua API.rst b/docs/Lua API.rst
index 6b8290d12..607dd424f 100644
--- a/docs/Lua API.rst	
+++ b/docs/Lua API.rst	
@@ -1307,7 +1307,8 @@ Units module
 
 * ``dfhack.units.isUndead(unit[,include_vamps])``
 
-  The unit is undead, but not a vampire.
+  The unit is undead. Pass ``true`` as the optional second parameter to
+  count vampires as undead.
 
 * ``dfhack.units.isGreatDanger(unit)``