How Can I use this function in a Query?

I have stored the following function in a module and I have tried to use it in a query as follows but I am getting a Data Type mismatch error when I run the query yet when I step through the function the result is a date field as expected. What am I doing wrong?

SELECT Tasks.*, Components.*, fStartOrder([StartOn],[DueDate]) AS Sort1, Nz([pkTaskType],"zzzz") AS Expr2, Nz([Priority],999) AS Expr3
FROM Tasks LEFT JOIN (Components LEFT JOIN tblTaskTypes ON Components.TaskType = tblTaskTypes.pkTaskType) ON Tasks.ID = Components.fkID
WHERE (((Components.Completed)<>True))
ORDER BY fStartOrder([StartOn],[DueDate]), Tasks.ID;



Function fStartOrder(StartOn As Date, DueDate As Date) As Date
    'Used in query to set the order of the StartOn value
    Dim StartOrder As Date
   
    If IsNull(StartOn) And IsNull(DueDate) Then
        StartOrder = Date
    ElseIf IsNull(DueDate) And Not IsNull(StartOn) Then
        StartOrder = StartOn
    ElseIf IsNull(StartOn) And Not IsNull(DueDate) Then
        StartOrder = DueDate
    ElseIf DueDate <= StartOn Then
        StartOrder = DueDate
    ElseIf StartOn < DueDate Then
        StartOrder = StartOn
    Else 'trap for missing entries
        StartOrder = #1/1/2001#
    End If
       
   fStartOrder = StartOrder
End Function
Rob4077Asked:
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Rey Obrero (Capricorn1)Connect With a Mentor Commented:
try changing this line

Function fStartOrder(StartOn As Date, DueDate As Date) As Date

with

Function fStartOrder(StartOn, DueDate) As Date


or leave as is
Function fStartOrder(StartOn As Date, DueDate As Date) As Date

but change the way you call the function in the query, like this

fStartOrder(nz([StartOn],#1/1/1900#),nz([DueDate],#1/1/1900#))
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Helen FeddemaCommented:
Or do some testing for valid dates in the function itself, using the IsDate function.  Just checking for the value not being Null may not be enough.
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Rob4077Author Commented:
Thanks for your suggestions. The fields DueDate and StartOn are both defined as date fields in the table so I thought that passing them to a function as date fields would work. Looks like it wasn't feeding information where the date field was null. As suggested by Capricorn1, changing it to a variant made it work (i.e. Function fStartOrder(StartOn, DueDate) As Date).
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