diff --git a/stim/image/image.h b/stim/image/image.h
index a18f179..df373f5 100644
--- a/stim/image/image.h
+++ b/stim/image/image.h
@@ -225,7 +225,7 @@ public:
 
 		for(size_t x = 0; x < X(); x++){
 			for(size_t y = 0; y < Y(); y++){
-				r.img[r.idx(x, y, c)] = img[idx(x, y, c)];
+				r.img[r.idx(x, y, 0)] = img[idx(x, y, c)];
 			}
 		}
 
--
libgit2 0.21.4