VBA Log function Runtime Error '5': Invalid procedure call or argument

I am trying to use the VBA Log function in an expression and I expect it to work on any number greater than 0. However, when I enter a relatively large number (e.g. 500 or 1,000), I get the error mentioned in the title. Here is the function:

Function f_x(x As Double) As Double

    f_x = 5 - x - Log(x)

End Function

I am using Option Explicit and I get the error when I enter x as any number greater than about 500.