Function using Haversine formula to calculate distance

Question

This is a function that I use to calculate distance from the latitude and longitude of two different locations. The earth radius is in Kilometer, and I have tried using both the "average" earth radius, and the calculated earth radius at the starting point. Either way, the answer should be in dozens of Kilometer but the results are over 8,000 and I cannot figure out why.

Anyone that might be able to help me figure out why the result is incorrect?

Public Function CalculateDistance(lat1 As Double, lon1 As Double, lat2 As Double, lon2 As Double) As Double
    

Function to calculate the straight-line distance between two locations
    Const EarthRadius As Double = 6387.59       '6371 is average  Radius of the Earth in kilometers
    
        If lon1 < 0 Then lon1 = 360 + lon1
        If lon2 < 0 Then lon2 = 360 + lon2

    ' Convert degrees to radians
    lat1 = lat1 * (3.14159265358979 / 180)
    lon1 = lon1 * (3.14159265358979 / 180)
    lat2 = lat2 * (3.14159265358979 / 180)
    lon2 = lon2 * (3.14159265358979 / 180)

    ' Calculate differences in coordinates
    Dim dLat As Double
    Dim dLon As Double
    dLat = lat2 - lat1
    dLon = lon2 - lon1

    ' Haversine formula to calculate distance
    Dim a As Double
    Dim c As Double
    a = Sin(dLat / 2) ^ 2 + Cos(lat1) * Cos(lat2) * Sin(dLon / 2) ^ 2
    c = 2 * Atn2(Sqr(a), Sqr(1 - a))
    CalculateDistance = EarthRadius * c

End Function

I was expecting 17.37 kilometers (give or take) and the result was over 8,600 kilometers.