diff --git a/src/experimental/filament/render_context_filament.cc b/src/experimental/filament/render_context_filament.cc
index c98129b0..6edce96e 100644
--- a/src/experimental/filament/render_context_filament.cc
+++ b/src/experimental/filament/render_context_filament.cc
@@ -137,8 +137,8 @@ void mjr_readPixels(unsigned char* rgb, float* depth, mjrRect viewport,
   g_filament_context->ReadPixels(viewport, rgb, depth);
 }
 
-void mjr_uploadFont(unsigned char* pixels, int width, int height, int bpp,
-                          int id, const mjrContext* con) {
+void mjr_uploadFont(const unsigned char* pixels, int width, int height, int bpp,
+                    int id, const mjrContext* con) {
   CheckFilamentContext();
   if (bpp != 4) {
     mju_error("Only 4bpp fonts are supported, got %d", bpp);
diff --git a/src/experimental/filament/render_context_filament.h b/src/experimental/filament/render_context_filament.h
index dcef98bf..7f913f5e 100644
--- a/src/experimental/filament/render_context_filament.h
+++ b/src/experimental/filament/render_context_filament.h
@@ -70,7 +70,7 @@ MJAPI void mjr_uploadMesh(const mjModel* m, const mjrContext* con, int meshid);
 
 MJAPI void mjr_uploadTexture(const mjModel* m, const mjrContext* con, int texid);
 
-void mjr_uploadFont(unsigned char* pixels, int width, int height, int bpp,
+void mjr_uploadFont(const unsigned char* pixels, int width, int height, int bpp,
                     int id, const mjrContext* con);
 
 MJAPI void mjr_setBuffer(int framebuffer, mjrContext* con);