diff --git a/spa/plugins/audioconvert/channelmix-ops.c b/spa/plugins/audioconvert/channelmix-ops.c
index d752c6fb1..5ec08a7f8 100644
--- a/spa/plugins/audioconvert/channelmix-ops.c
+++ b/spa/plugins/audioconvert/channelmix-ops.c
@@ -168,14 +168,19 @@ static int make_matrix(struct channelmix *mix)
 	float llev = 0.5f;
 	float max = 0.0f;
 
+	spa_log_debug(mix->log, "src-mask:%08"PRIx64" dst-mask:%08"PRIx64,
+			src_mask, dst_mask);
+
+	if ((src_mask & _MASK(MONO)) ==  _MASK(MONO))
+		src_mask = _MASK(FC);
+	if ((dst_mask & _MASK(MONO)) ==  _MASK(MONO))
+		dst_mask = _MASK(FC);
+
 	for (i = 0; i < NUM_CHAN; i++) {
 		if (src_mask & dst_mask & (1ULL << (i + 2)))
 			matrix[i][i]= 1.0f;
 	}
 
-	if ((dst_mask & _MASK(MONO)) ==  _MASK(MONO))
-		dst_mask = _MASK(FC);
-
 	unassigned = src_mask & ~dst_mask;
 
 	spa_log_debug(mix->log, "unassigned %08" PRIx64, unassigned);
diff --git a/spa/plugins/audioconvert/channelmix.c b/spa/plugins/audioconvert/channelmix.c
index c79724253..53c09f46d 100644
--- a/spa/plugins/audioconvert/channelmix.c
+++ b/spa/plugins/audioconvert/channelmix.c
@@ -211,9 +211,9 @@ static int setup_convert(struct impl *this,
 	for (i = 0, dst_mask = 0; i < dst_chan; i++)
 		dst_mask |= 1UL << dst_info->info.raw.position[i];
 
-	if (src_mask & 1)
+	if (src_mask & 1 || src_chan == 1)
 		src_mask = default_mask(src_chan);
-	if (dst_mask & 1)
+	if (dst_mask & 1 || dst_chan == 1)
 		dst_mask = default_mask(dst_chan);
 
 	spa_log_info(this->log, NAME " %p: %s/%d@%d->%s/%d@%d %08"PRIx64":%08"PRIx64, this,