C# - Random String Function ?

I've made a function in C# to create a random string, but I wanted to convert it to VB.NET, unfortunately my knowledge of Visual Basic is much less than my knowledge of C#.

Here is my VB.NET function:

CODE:

As you will see, its not much different from my C# version, except that since VB can take an optional parameter, I allow the user to select what characters to use in the string, or just use the default ones.

Here's the C# version of my function:

CODE:

Again, there's not much different, but the part I'm really struggling on is the conversion between what is the 12th line of VB code, and the 13th line of C# code. I didn't really know how to convert it to VB.NET (as I said, my knowledge of it is very limited), so I used an online converter. The result of the online converter runs with no errors, however when I try to call the function, no string appears.

In short, this C# code works fine:

CODE:

However, this VB.NET code doesn't work:

CODE:

How could I fix my VB.NET code?