--- res_fax.c-orig	2013-12-05 21:00:42.521150823 +0100
+++ res_fax.c	2013-12-05 21:04:48.370712639 +0100
@@ -3365,7 +3365,7 @@
 		return f;
 	}
 
-	if (f->frametype == AST_FRAME_VOICE) {
+	if (f->frametype == AST_FRAME_VOICE && faxdetect->dsp) {
 		f = ast_dsp_process(chan, faxdetect->dsp, f);
 		if (f->frametype == AST_FRAME_DTMF) {
 			result = f->subclass.integer;