I have an IRIS persistent class with a %Stream property whose value is a JSON object. I'd like to use a SQL trigger to pull some value out of the JSON and persist it in another property, for easy querying and indexing. See below for a minimal example:

Class PAB.DebugStream Extends %Persistent
{

Property Contents As %Stream.GlobalCharacter;
Property msg As %String;
ClassMethod InsertRow()
{
    set stream = ##class(%Stream.GlobalCharacter).%New()
    $$$ThrowOnError(stream.Write({"msg":"hello world!"}))
    &sql(insert into PAB.DebugStream (Contents) values (:stream))
    $$$ThrowSQLIfError(SQLCODE, %msg)
}

Trigger ExtractKeys [ Event = INSERT/UPDATE, Foreach = row/object, Time = AFTER ]
{
    new contentsJSON, id, msg
    if {Contents*C} {
        set contentsJSON = {}.%FromJSON({Contents})
        set id = {ID}
        set msg = contentsJSON.msg
        &sql(update PAB.DebugStream set msg = :msg where Id = :id)
        $$$ThrowSQLIfError(SQLCODE, %msg)
    }
}
}

However, the SQL insert fails in the trigger with a JSON parsing message like this:

<THROW>InsertRow+9^PAB.DebugStream.1 *%Exception.SQL -415 -415 InsertRow+9^PAB.DebugStream.1 Error occurring during INSERT in table 'PAB.DebugStream':  $ZE=<THROW>%FromJSON+22^%Library.DynamicAbstractObject.1 *%Exception.General Parsing error 3 Line 1 Offset 1

If I log the value of {Contents} in the trigger, it's not the stream object as I expect but an integer value.

How can I get the new value of a %Stream property in a SQL trigger?

I'm also open to suggestions about better ways to persist/index specific paths in a JSON property.