Script error in CLI but not ISE???

I get an error when running a script via the cli, but not Powershell ISE! I do not understand why.

 ERROR: [06/09/2016 07:57:26] System.Management.Automation.RuntimeException: Method invocation failed because [System.Object] does not contain a method named 'op_Addition'.
       at System.Management.Automation.ExceptionHandlingOps.CheckActionPreference(FunctionContext funcContext, Exception exception)
       at System.Management.Automation.Interpreter.ActionCallInstruction`2.Run(InterpretedFrame frame)
       at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)
       at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)

The code snippet:

    $logMessages = @()
    ...
    Function Timestamp-Message([string] $message){
        $msg = New-Object System.Object
        $msg | Add-Member -type NoteProperty -name TimeStamp -value $(Get-Date)
        $msg | Add-Member -type NoteProperty -name Message -value $message
        return $msg
    }
    ...
    Process {
        Try {
            ...
            $logMessages += Timestamp-Message($message)

The error occurs on this last statement. And this exact statement is executed a few lines before with no exception, storing the object as the first member of the array.