diff --git a/channels/chan_sip.c b/channels/chan_sip.c
index 5418498fd4..10fbcf5ada 100644
--- a/channels/chan_sip.c
+++ b/channels/chan_sip.c
@@ -6916,8 +6916,9 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req, int t38action
 		int text = FALSE;
 
 		numberofports = 1;
-		if ((sscanf(m, "audio %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2) ||
-		    (sscanf(m, "audio %d RTP/AVP %n", &x, &len) == 1)) {
+		len = -1;
+		if ((sscanf(m, "audio %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2 && len > 0) ||
+		    (sscanf(m, "audio %d RTP/AVP %n", &x, &len) == 1 && len > 0)) {
 			audio = TRUE;
 			numberofmediastreams++;
 			/* Found audio stream in this media definition */
@@ -6932,8 +6933,8 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req, int t38action
 					ast_verbose("Found RTP audio format %d\n", codec);
 				ast_rtp_set_m_type(newaudiortp, codec);
 			}
-		} else if ((sscanf(m, "video %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2) ||
-		    (sscanf(m, "video %d RTP/AVP %n", &x, &len) == 1)) {
+		} else if ((sscanf(m, "video %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2 && len > 0) ||
+		    (sscanf(m, "video %d RTP/AVP %n", &x, &len) == 1 && len >= 0)) {
 			video = TRUE;
 			p->novideo = FALSE;
 			numberofmediastreams++;
@@ -6948,8 +6949,8 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req, int t38action
 					ast_verbose("Found RTP video format %d\n", codec);
 				ast_rtp_set_m_type(newvideortp, codec);
 			}
-		} else if ((sscanf(m, "text %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2) ||
-		    (sscanf(m, "text %d RTP/AVP %n", &x, &len) == 1)) {
+		} else if ((sscanf(m, "text %d/%d RTP/AVP %n", &x, &numberofports, &len) == 2 && len > 0) ||
+		    (sscanf(m, "text %d RTP/AVP %n", &x, &len) == 1 && len > 0)) {
 			text = TRUE;
 			p->notext = FALSE;
 			numberofmediastreams++;
@@ -6964,8 +6965,8 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req, int t38action
 					ast_verbose("Found RTP text format %d\n", codec);
 				ast_rtp_set_m_type(newtextrtp, codec);
 			}
-		} else if (p->udptl && ( (sscanf(m, "image %d udptl t38%n", &x, &len) == 1) || 
-		 (sscanf(m, "image %d UDPTL t38%n", &x, &len) == 1) )) {
+		} else if (p->udptl && ( (sscanf(m, "image %d udptl t38%n", &x, &len) == 1 && len > 0) || 
+			(sscanf(m, "image %d UDPTL t38%n", &x, &len) == 1 && len > 0) )) {
 			if (debug)
 				ast_verbose("Got T.38 offer in SDP in dialog %s\n", p->callid);
 			udptlportno = x;