Index: src/natools-smaz_implementations-base_64.adb
==================================================================
--- src/natools-smaz_implementations-base_64.adb
+++ src/natools-smaz_implementations-base_64.adb
@@ -240,11 +240,11 @@
 
                Tools.Encode_Double
                  (Input (Index .. Index + 1),
                   Tools.Double_Byte_Padding (Extra_Blocks mod 4),
                   Output, Offset);
-               Index := Index + 1;
+               Index := Index + 2;
 
                if Extra_Blocks > 0 then
                   Tools.Encode
                     (Input (Index .. Index + Extra_Blocks * 3 - 1),
                      Output, Offset);