vba mid function weird error

Hi,

So I'm trying to get the last 9 numbers from an integer by using the mid function.
In this example It's to calculate a French TVA number:

Private Sub TextBoxBtw_Change()
    CommandButtonBtw.Enabled = False
    
    Dim TrimmedResult As String
    Dim IsResultNumeric As Boolean
    
    TrimmedResult = Replace(TextBoxBtw.Text, " ", "")
    TrimmedResult = Replace(UCase(TrimmedResult), "FR", "")
    IsResultNumeric = IsNumeric(TrimmedResult)
    If Len(TrimmedResult) = 11 And IsResultNumeric = True Then
        Dim controlNumber As Integer
        Dim sirenNumber As Integer
        
        controlNumber = Mid(TrimmedResult, 1, 2)
        sirenNumber = Mid(TrimmedResult, 3)
        If controlNumber = (12 + 3 * (sirenNumber Mod 97)) Mod 97 Then
            CommandButtonBtw.Enabled = True
        End If
    End If
End Sub

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But for some reason it gives me a "loop error?" on sirenNumber = Mid(TrimmedResult, 3)

The weird part is when I type 6 instead of 3 it works and give me the last 5 numbers.
BUT when I type for example Mid(TrimmedResult, 3, 5) which would give me also 5 numbers it doesnt work... only 4 in length work when I indicate the length.

Why oh why does vba hate me?
Any ideas?