Conversion from empty Date to type 'Date' is not valid - vb.net

Question

My Function works fine until I return an empty or NULL result, then i get the error: Conversion from string "" to type 'Date' is not valid.
VB.NET:
Public Function GetNextWaitListed(ByVal ClassName As String, ByVal ClassDate As Date, ByVal ClassTime As String) As String
Dim connStr As String = ConfigurationManager.AppSettings.Get("TechTrainingConn")
Dim conn As New Data.OleDb.OleDbConnection(connStr)
Try
conn.Open()
Dim sql As String = "SELECT Min(SubmitTime) FROM [EnrollmentsTbl]" & _
" WHERE [ClassName] = """ & ClassName & """" & _
" AND [ClassDate] = #" & ClassDate & "#" & _
" AND [ClassTime] = """ & ClassTime & """" & _
" AND [Waitlisted] = True" & _
" AND [Completed] = False" & _
" AND [Enrolled] = True"
Dim comm As New Data.OleDb.OleDbCommand(sql, conn)
Dim result As Date = comm.ExecuteScalar()
If Not String.IsNullOrEmpty(result) Then
Return result
End If
Catch ex As Exception
Response.Write(ex)
Finally
conn.Close()
End Try
End Function
I have tried the following but am a little confused since I returning a date
If Not String.IsNullOrEmpty(result) Then
Dim oDate As DateTime = Convert.ToDateTime(result)
Return oDate
End If

Answer 1

The best solution for me was to return into object and if object was empty do the else with dummy date
Public Function GetNextWaitListed(ByVal ClassName As String, ByVal ClassDate As Date, ByVal ClassTime As String) As String
Dim connStr As String = ConfigurationManager.AppSettings.Get("TechTrainingConn")
Dim conn As New Data.OleDb.OleDbConnection(connStr)
Try
conn.Open()
Dim sql As String = "SELECT Min(SubmitTime) FROM [EnrollmentsTbl]" & _
" WHERE [ClassName] = """ & ClassName & """" & _
" AND [ClassDate] = #" & ClassDate & "#" & _
" AND [ClassTime] = """ & ClassTime & """" & _
" AND [Waitlisted] = True" & _
" AND [Completed] = False" & _
" AND [Enrolled] = True"
Dim comm As New Data.OleDb.OleDbCommand(sql, conn)
Dim Obj As Object = comm.ExecuteScalar()
If (Obj IsNot Nothing) AndAlso (Obj IsNot DBNull.Value) Then
Dim matches As String = Obj.ToString
Dim result As Date = Convert.ToDateTime(matches)
Return result
Else
Dim result As Date = Convert.ToDateTime("01/01/1900")
Return result
End If
Catch ex As Exception
Response.Write(ex)
If Not conn Is Nothing Then
conn.Close()
End If
Finally
conn.Close()
End Try
End Function