Index: src/natools-s_expressions-encodings.adb
==================================================================
--- src/natools-s_expressions-encodings.adb
+++ src/natools-s_expressions-encodings.adb
@@ -368,13 +368,12 @@
          if Result (I) = Plus then
             Result (I) := Digit_62;
          elsif Result (I) = Slash then
             Result (I) := Digit_63;
          elsif Result (I) = Base64_Filler then
-            for J in I + 1 .. Result'Last loop
-               pragma Assert (Result (J) = Base64_Filler);
-            end loop;
+            pragma Assert (Result (I + 1 .. Result'Last)
+              = (I + 1 .. Result'Last => Base64_Filler));
             Last := I - 1;
             exit;
          end if;
       end loop;