diff --git a/src/dwarfs/worker_group.cpp b/src/dwarfs/worker_group.cpp
index bcf0c5d7..589af68a 100644
--- a/src/dwarfs/worker_group.cpp
+++ b/src/dwarfs/worker_group.cpp
@@ -35,6 +35,7 @@
 #include <folly/String.h>
 #include <folly/portability/PThread.h>
 #include <folly/portability/Windows.h>
+#include <folly/system/HardwareConcurrency.h>
 #include <folly/system/ThreadName.h>
 
 #include "dwarfs/error.h"
@@ -102,7 +103,7 @@ class basic_worker_group final : public worker_group::impl, private Policy {
       , pending_(0)
       , max_queue_len_(max_queue_len) {
     if (num_workers < 1) {
-      DWARFS_THROW(runtime_error, "invalid number of worker threads");
+      num_workers = std::max(folly::hardware_concurrency(), 1u);
     }
 
     if (!group_name) {