Fehler beim Kompilieren

Function Nußelt(Nu_a As Double, Re_a As Double, Pr_a As Double, C_o As Double, m As Double, n  _
As Double, D_to As Double, D_ai As Double, fa_Ring_Berechnung As Double) As Double
Select Case Nu_a
Case Is = 1                                                             'Rohr mit Draht
Nußelt = C_o * Re_a ^ m * Pr_a ^ n
Case Is = 2                                                             'Rohr ohne  _
Draht
Nußelt = ((fa_Ring_Berechnung / 2) * Re_a * Pr_a) / (1.07 + 12.7 * ( _
fa_Ring_Berechnung / 2) ^ 0.5 * ( Pr_a ^(2/3) -1)
Case Is = 3                                                             'Kreisring
Nußelt =
End Function