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
 
diff --git a/src/hathreads.c b/src/hathreads.c
index 39ff7e8..a21bddd 100644
--- a/src/hathreads.c
+++ b/src/hathreads.c
@@ -108,13 +108,22 @@
 __attribute__((constructor))
 static void __hathreads_init(void)
 {
+	char *ptr = NULL;
+
+	if (MAX_THREADS < 1 || MAX_THREADS > LONGBITS) {
+		ha_alert("MAX_THREADS value must be between 1 and %d inclusive; "
+		         "HAProxy was built with value %d, please fix it and rebuild.\n",
+			 LONGBITS, MAX_THREADS);
+		exit(1);
+	}
+	memprintf(&ptr, "Built with multi-threading support (MAX_THREADS=%d).", MAX_THREADS);
+	hap_register_build_opts(ptr, 1);
+
 #if defined(DEBUG_THREAD) || defined(DEBUG_FULL)
 	memset(lock_stats, 0, sizeof(lock_stats));
 #endif
 }
 
-REGISTER_BUILD_OPTS("Built with multi-threading support.");
-
 #else
 
 REGISTER_BUILD_OPTS("Built without multi-threading support (USE_THREAD not set).");