diff --git a/data/tools/wmlindent b/data/tools/wmlindent
index 9ae64eab12d..a3ded9c4555 100755
--- a/data/tools/wmlindent
+++ b/data/tools/wmlindent
@@ -68,7 +68,7 @@ opener_prefixes = ["{FOREACH "]
 
 def is_directive(str):
     "Identify things that shouldn't be indented."
-    for prefix in ("#ifdef", "#ifndef", "#else", "#endif", "#define", "#enddef"):
+    for prefix in ("#ifdef", "#ifndef", "#else", "#endif", "#define", "#enddef", "#undef"):
         if str.startswith(prefix):
             return True
     return False