diff --git a/data/tools/wmllint b/data/tools/wmllint
index 959976ba075..13d455ff0a2 100755
--- a/data/tools/wmllint
+++ b/data/tools/wmllint
@@ -547,6 +547,15 @@ def validate_on_pop(tagstack, closer, filename, lineno):
     if closer == "unit" and attributes.get("id") is not None and attributes.get("type") is not None and attributes.get("side") is None and not "side" in ancestors:
         print '"%s", line %d: unit declaration without side attribute' % \
               (filename, lineno)
+    if closer == "filter_side":
+        ancestor = False
+        if "gold" in ancestors:
+            ancestor = "gold"
+        elif "modify_ai" in ancestors:
+            ancestor = "modify_ai"
+        if ancestor:
+            print '"%s", line %d: %s should have an inline SSF instead of using [filter_side]' % \
+                (filename, lineno, ancestor)
 
 def within(tag):
     "Did the specified tag lead one of our enclosing contexts?"