Gnat bug or mistaken program?

[Richard Iswara <haujekchifan@gmail.com>]

I get this error on my program from the menu build => check semantic
exponent must be of type Natural, found type "Standard.Float".

Should not 10 ** (log10 a + log10 b) = a*b?

This is my compiler build:

GNAT Studio Community 2020 (20200427) hosted on x86_64-pc-mingw32
GNAT Community 2020 (20200429-93) targetting x86_64-pc-mingw32
SPARK Community 2020 (20200429)

Here is the relevant part of program:

with Ada.Numerics.Elementary_Functions;  
use Ada.Numerics;

Counter : Integer := 1;
Logs : Float := 0.0;
Multiples : Float;

inside some loop:

Logs := Elementary_Functions.Log (Float(Counter), 10.0) + Logs;
Multiples := 10.0 ** Logs;  => this is where it fails

I checked the Ada.Numerics.Generic_Elementary_Functions specifications
(since Ada.Numerics.Elementary_Functions is just an instance of the generics)
and it said:

   function "**" (Left, Right : Float_Type'Base) return Float_Type'Base with
     Pre  => (if Left = 0.0 then Right > 0.0) and Left >= 0.0,
     Post => "**"'Result >= 0.0
       and then (if Right = 0.0 then "**"'Result = 1.0)
       and then (if Right = 1.0 then "**"'Result = Left)
       and then (if Left  = 1.0 then "**"'Result = 1.0)
       and then (if Left  = 0.0 then "**"'Result = 0.0);

So the question is this a Gnat bug, wrong function used, faulty logic on me or
I am declaring it wrong?