diff --git a/video/out/opengl/ra.h b/video/out/opengl/ra.h
index 156dce9d83..f0798f018c 100644
--- a/video/out/opengl/ra.h
+++ b/video/out/opengl/ra.h
@@ -398,7 +398,8 @@ struct ra_fns {
     // Hint that possibly queued up commands should be sent to the GPU. Optional.
     void (*flush)(struct ra *ra);
 
-    // Optional.
+    // Associates a marker with any past error messages, for debugging
+    // purposes. Optional.
     void (*debug_marker)(struct ra *ra, const char *msg);
 };