error in mule 4 "You cannot compare a value of type :Null" - mule-component

Question

I am trying to find the max of date from json data. But i am getting below error
Message : "You cannot compare a value of type :Null
Trace:
at reduce (Unknown)
at dw::Core::maxBy (line: 5535, column: 3)
at main (line: 1, column: 248)" evaluating expression:
"%dw 2.0 output application/json --- { Value2: ( if (vars.data.Value1 as String != "")
(payload maxBy((item) -> item.startDate)).startDate default vars.data.Value1
else "" ), RCount: sizeOf(payload) }".
Error type : MULE:EXPRESSION
Element : executeInterface/processors/8 # gg:gg.xml:121 (interface)
Element XML : <set-variable value="#[%dw 2.0 output application/json ---
{ Value2: ( if (vars.data.Value1 as String != "")
(payload maxBy((item) -> item.startDate)).startDate default vars.data.Value1
else "" ), RCount: sizeOf(payload) }]" doc:name="interface" doc:id="2c140185-2fc5-4e11-9b78-96fc2ddcfa2f" variableName="interface"></set-variable>
(set debug level logging or '-Dmule.verbose.exceptions=true' for everything).
The logic written in set variable component is
%dw 2.0
output application/json
---
{
Value2:
( if (vars.data.Value1 as String != "")
(payload maxBy((item) -> item.startDate)).startDate default vars.data.Value1
else ""
),
RCount: sizeOf(payload)
}
while running in the debug mode, found the problem is in maxby statement.
Please suggest.how to fix this issue

Answer 1

The error indicates that something is null when trying to compare. Given that you mentioned the error is at maxBy(), it is probable the the attribute startDate is null or not present in one of the elements. You should show also the values of the transformation so we could confirm it.
UPDATE: I can reproduce the error if the startDate attribute is not present or if it is misspelled. For example if I misspell item.StartDate (wrong uppercase 'S') it produces the error. Make sure the payload and script match exactly.