Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c (revision 177)
+++ channels/chan_sip.c (working copy)
@@ -2976,6 +2976,11 @@
                return &null_frame;
        if (p->owner) {
                /* We already hold the channel lock */
+               if (!f) {
+                       /* We have no RTP allocated for this channel */
+                       return &null_frame;
+               }
+
                if (f->frametype == AST_FRAME_VOICE) {
                        if (f->subclass != p->owner->nativeformats) {
                                ast_log(LOG_DEBUG, "Oooh, format changed to %d\n", f->subclass);