diff --git a/dlib/threads/thread_pool_extension.cpp b/dlib/threads/thread_pool_extension.cpp
index 0544f41a02a80b0cde426215fc225d135cdda336..1434b221f650a5e6590db5b41378830f70ec8e25 100644
--- a/dlib/threads/thread_pool_extension.cpp
+++ b/dlib/threads/thread_pool_extension.cpp
@@ -216,7 +216,7 @@ namespace dlib
         uint64 id
     ) const
     {
-        return id%tasks.size();
+        return static_cast<unsigned long>(id%tasks.size());
     }
 
 // ----------------------------------------------------------------------------------------