diff --git a/backend/mustek_pp.c b/backend/mustek_pp.c
index 7fb87b0d5..64eef03d4 100644
--- a/backend/mustek_pp.c
+++ b/backend/mustek_pp.c
@@ -523,7 +523,7 @@ send_voltages (Mustek_PP_Device * dev)
 
 }
 
-int
+static int
 compar (const void *a, const void *b)
 {
   return (signed int) (*(const SANE_Byte *) a) -