diff --git a/df/parallel.c b/df/parallel.c
index ef3137563..047735ef3 100644
--- a/df/parallel.c
+++ b/df/parallel.c
@@ -194,9 +194,15 @@ worker_thread (void *thread_data_vp)
     guestfs_set_verbose (g, thread_data->verbose);
 
     /* Do work. */
-    if (thread_data->work (g, i, fp) == -1)
+    if (thread_data->work (g, i, fp) == -1) {
       thread_data->r = -1;
 
+      if (thread_data->verbose)
+        fprintf (stderr,
+                 "parallel: thread %zu work function returned an error\n",
+                 thread_data->thread_num);
+    }
+
     fclose (fp);
     guestfs_close (g);