diff --git a/ppl/src/main/antlr/OpenSearchPPLParser.g4 b/ppl/src/main/antlr/OpenSearchPPLParser.g4
index 5eed9db26a..e95a933c67 100644
--- a/ppl/src/main/antlr/OpenSearchPPLParser.g4
+++ b/ppl/src/main/antlr/OpenSearchPPLParser.g4
@@ -863,6 +863,7 @@ keywordsCanBeId
    | DEDUP
    | SORT
    | EVAL
+   | FILLNULL
    | HEAD
    | TOP
    | RARE