diff --git a/gst-libs/gst/sdp/gstsdpmessage.c b/gst-libs/gst/sdp/gstsdpmessage.c
index 7e3bbb8343..9231cb7b1a 100644
--- a/gst-libs/gst/sdp/gstsdpmessage.c
+++ b/gst-libs/gst/sdp/gstsdpmessage.c
@@ -598,7 +598,7 @@ static const guchar acceptable[96] = {
 
 static const gchar hex[16] = "0123456789ABCDEF";
 
-#define ACCEPTABLE_CHAR(a) ((a)>=32 && (a)<128 && acceptable[(a)-32])
+#define ACCEPTABLE_CHAR(a) (((guchar)(a))>=32 && ((guchar)(a))<128 && acceptable[(((guchar)a))-32])
 
 /**
  * gst_sdp_message_as_uri: