A blank input to a InputBox (method) runs the alert of typing formula in a cell with error

Hi, everybody,
I want a very simple InputBox, but I want it to be with the InputBox method, to validate the input as numeric and evaluate easyly when it is cancelled by the user. By the way, then I'll do one more validation to the input value, no problem with that.

When I clic OK with nothing in the entry field of the InputBox dialog, I get the same alert you get when you type a formula which contains errors in a cell:
"The formula you typed contains an error.

• For information about fixing...
• To get assistance in...
• If you are not trying..."

It supposed that a blanck entry will yield a 0-length string, perhaps a vbNullString. The variable getting the value is variant. Parameter "Type" is 1.
I made trials with/without default value. And don't want to go to the InputBox function.
Why the hell Excel-VBA thinks anyone is typing anything into a cell?

Attached a sample to reproduce this behavior. Wondering to know from you guys what stupid mistake I'm doing.
Thank you.