diff --git a/src/pulsecore/source-output.c b/src/pulsecore/source-output.c
index 1e08a03f5..091d7f98d 100644
--- a/src/pulsecore/source-output.c
+++ b/src/pulsecore/source-output.c
@@ -243,7 +243,8 @@ int pa_source_output_new(
          * to using the sample spec and channel map after all decisions w.r.t.
          * routing are complete. */
         pa_idxset *tmp = pa_idxset_new(NULL, NULL);
-        pa_format_info *f = pa_format_info_from_sample_spec(&data->sample_spec, &data->channel_map);
+        pa_format_info *f = pa_format_info_from_sample_spec(&data->sample_spec,
+                data->channel_map_is_set ? &data->channel_map : NULL);
         pa_idxset_put(tmp, f, NULL);
         pa_source_output_new_data_set_formats(data, tmp);
     }