diff --git a/src/doc/sort_palette.cpp b/src/doc/sort_palette.cpp
index b4c529295..29d892eb1 100644
--- a/src/doc/sort_palette.cpp
+++ b/src/doc/sort_palette.cpp
@@ -1,5 +1,5 @@
 // Aseprite Document Library
-// Copyright (c) 2001-2015 David Capello
+// Copyright (c) 2001-2017 David Capello
 //
 // This file is released under the terms of the MIT license.
 // Read LICENSE.txt for more information.
@@ -121,7 +121,9 @@ struct PalEntryWithIndexPredicate {
 
 } // anonymous namespace
 
-Remap sort_palette(Palette* palette, SortPaletteBy channel, bool ascending)
+Remap sort_palette(const Palette* palette,
+                   const SortPaletteBy channel,
+                   const bool ascending)
 {
   std::vector<PalEntryWithIndex> tmp(palette->size());
   for (int i=0; i<palette->size(); ++i) {
diff --git a/src/doc/sort_palette.h b/src/doc/sort_palette.h
index 73fdec73c..bb38d8a55 100644
--- a/src/doc/sort_palette.h
+++ b/src/doc/sort_palette.h
@@ -1,5 +1,5 @@
 // Aseprite Document Library
-// Copyright (c) 2001-2015 David Capello
+// Copyright (c) 2001-2017 David Capello
 //
 // This file is released under the terms of the MIT license.
 // Read LICENSE.txt for more information.
@@ -26,7 +26,9 @@ namespace doc {
   };
 
   // Creates a Remap to sort the palette. It doesn't apply the remap.
-  Remap sort_palette(Palette* palette, SortPaletteBy channel, bool ascending);
+  Remap sort_palette(const Palette* palette,
+                     const SortPaletteBy channel,
+                     const bool ascending);
 
 } // namespace doc