I am trying to define a derivative function for solving a system of differential equations, however when I run the actual macro that calls up this sub routine, i keep getting Run-time error 5: Invalid procedure call or argument. This error occurs in the If statement when x is not greater than 1 and Qv is to be calculated using the equation provided. While stepping into to debug, there are values for all of the variables, but it gives me the error and I have no idea why. Can someone help please?


Sub Derivs(x As Double, y() As Double, dydx() As Double)

Const g As Double = 32.1740485564
Const Hr As Double = 100
Const h0 As Double = 80
Const fm As Double = 0.024
Const L As Double = 1500
Const dp As Double = 2
Const tc As Double = 5
Const k As Double = 25.7
Const Di As Double = 5

Dim u0 As Double
Dim Qv As Double
Dim Qv0 As Double
Dim hstar As Double

u0 = ((g * h0 / ((1 / 2) * fm * (L / dp))) * ((Hr / h0) - 1)) ^ (1 / 2)

Qv0 = (u0 * 3.14 * Di ^ 2) / 4

hstar = h0 - (Qv0 / k) ^ 2

If x >= 1 Then
Qv = 0
Else
Qv = k * (h0 ^ 0.5) * (1 - x) * (y(1) - hstar / h0) ^ 0.5
End If

dydx(0) = ((tc * g * h0) / (L * u0)) * (((Hr / h0) - y(1)) - ((Hr / h0) - 1) * y(0) * Abs(y(0)))

dydx(1) = ((dp / Di) ^ 2) * (u0 * tc / h0) * y(0) - ((4 * Qv * tc) / (3.14 * h0 * Di ^ 2))

End Sub