diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index e1b49d57..3722200 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -649,7 +649,7 @@ _outMergeJoin(StringInfo str, const MergeJoin *node)
 
 	appendStringInfoString(str, " :mergeNullsFirst");
 	for (i = 0; i < numCols; i++)
-		appendStringInfo(str, " %d", (int) node->mergeNullsFirst[i]);
+		appendStringInfo(str, " %s", booltostr(node->mergeNullsFirst[i]));
 }
 
 static void