diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
index 3d2dbac24eae8..d9784882de805 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
@@ -98,4 +98,6 @@ case class MaxOf(left: Expression, right: Expression) extends Expression {
   override def dataType = left.dataType
 
   override def eval(input: Row): Any = ???
+
+  override def toString = s"MaxOf($left, $right)"
 }