diff --git a/panda/src/pnmimage/pnmImage.cxx b/panda/src/pnmimage/pnmImage.cxx
index eb4056abd5..da7b0f5f62 100644
--- a/panda/src/pnmimage/pnmImage.cxx
+++ b/panda/src/pnmimage/pnmImage.cxx
@@ -199,7 +199,7 @@ alpha_fill_val(xelval alpha) {
 //               Input must be a color image.
 ////////////////////////////////////////////////////////////////////
 void PNMImage::
-remix_channels(LMatrix4f &conv) {
+remix_channels(const LMatrix4f &conv) {
   int nchannels = get_num_channels();
   nassertv((nchannels >= 3) && (nchannels <= 4));
   for (int y = 0; y < get_y_size(); y++) {
diff --git a/panda/src/pnmimage/pnmImage.h b/panda/src/pnmimage/pnmImage.h
index 14c89951c5..f114615fd4 100644
--- a/panda/src/pnmimage/pnmImage.h
+++ b/panda/src/pnmimage/pnmImage.h
@@ -83,7 +83,7 @@ PUBLISHED:
   INLINE void fill(double red, double green, double blue);
   INLINE void fill(double gray = 0.0);
 
-  void remix_channels(LMatrix4f &conv);
+  void remix_channels(const LMatrix4f &conv);
 
   void fill_val(xelval red, xelval green, xelval blue);
   INLINE void fill_val(xelval gray = 0);