diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 8e96e37..019d749 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -168,8 +168,11 @@
 #include <pthread.h>
 #include <import/plock.h>
 
+#ifndef MAX_THREADS
 #define MAX_THREADS LONGBITS
-#define MAX_THREADS_MASK ((unsigned long)-1)
+#endif
+
+#define MAX_THREADS_MASK (~0UL >> (LONGBITS - MAX_THREADS))
 
 #define __decl_hathreads(decl) decl