diff --git a/rem/vidmix/vidmix.c b/rem/vidmix/vidmix.c
index 7aae7f5dc..29cec084a 100644
--- a/rem/vidmix/vidmix.c
+++ b/rem/vidmix/vidmix.c
@@ -89,7 +89,7 @@ static void source_destructor(void *arg)
 
 
 static inline void source_mix(struct vidframe *mframe,
-			      struct vidframe *frame_src,
+			      const struct vidframe *frame_src,
 			      unsigned n, unsigned rows, unsigned idx,
 			      bool focus, bool focus_this, bool focus_full)
 {