diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 35b6e6eac..068b2a153 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -28,6 +28,12 @@
 extern THREAD_LOCAL unsigned int tid;     /* The thread id */
 extern THREAD_LOCAL unsigned long tid_bit; /* The bit corresponding to the thread id */
 
+/* Note about all_threads_mask :
+ *    - with threads support disabled, this symbol is defined as zero (0UL).
+ *    - with threads enabled, this variable is never zero, it contains the mask
+ *      of enabled threads. Thus if only one thread is enabled, it equals 1.
+ */
+
 #ifndef USE_THREAD
 
 #define MAX_THREADS 1
@@ -85,7 +91,7 @@ extern THREAD_LOCAL unsigned long tid_bit; /* The bit corresponding to the threa
 
 #define HA_BARRIER() do { } while (0)
 
-#define THREAD_SYNC_INIT(m)  do { /* do nothing */ } while(0)
+#define THREAD_SYNC_INIT()   do { /* do nothing */ } while(0)
 #define THREAD_SYNC_ENABLE() do { /* do nothing */ } while(0)
 #define THREAD_WANT_SYNC()   do { /* do nothing */ } while(0)
 #define THREAD_ENTER_SYNC()  do { /* do nothing */ } while(0)
@@ -244,7 +250,7 @@ static inline void __ha_barrier_full(void)
 
 #define HA_BARRIER() pl_barrier()
 
-#define THREAD_SYNC_INIT(m)   thread_sync_init(m)
+#define THREAD_SYNC_INIT()    thread_sync_init()
 #define THREAD_SYNC_ENABLE()  thread_sync_enable()
 #define THREAD_WANT_SYNC()    thread_want_sync()
 #define THREAD_ENTER_SYNC()   thread_enter_sync()
@@ -252,7 +258,7 @@ static inline void __ha_barrier_full(void)
 #define THREAD_NO_SYNC()      thread_no_sync()
 #define THREAD_NEED_SYNC()    thread_need_sync()
 
-int  thread_sync_init(int nbthread);
+int  thread_sync_init();
 void thread_sync_enable(void);
 void thread_want_sync(void);
 void thread_enter_sync(void);
@@ -877,5 +883,6 @@ static inline void __ha_compiler_barrier(void)
 
 /* Dummy I/O handler used by the sync pipe.*/
 void thread_sync_io_handler(int fd);
+int parse_nbthread(const char *arg, char **err);
 
 #endif /* _COMMON_HATHREADS_H */
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 9841e2dc6..803a9189b 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -1190,18 +1190,10 @@ int cfg_parse_global(const char *file, int linenum, char **args, int kwm)
 			err_code |= ERR_ALERT | ERR_FATAL;
 			goto out;
 		}
-		global.nbthread = atol(args[1]);
-#ifndef USE_THREAD
-		if (global.nbthread > 1) {
-			ha_alert("HAProxy is not compiled with threads support, please check build options for USE_THREAD.\n");
-			global.nbthread = 1;
-			err_code |= ERR_ALERT | ERR_FATAL;
-			goto out;
-		}
-#endif
-		if (global.nbthread < 1 || global.nbthread > MAX_THREADS) {
-			ha_alert("parsing [%s:%d] : '%s' must be between 1 and %d (was %d).\n",
-				 file, linenum, args[0], MAX_THREADS, global.nbthread);
+		global.nbthread = parse_nbthread(args[1], &errmsg);
+		if (!global.nbthread) {
+			ha_alert("parsing [%s:%d] : '%s' %s.\n",
+				 file, linenum, args[0], errmsg);
 			err_code |= ERR_ALERT | ERR_FATAL;
 			goto out;
 		}
diff --git a/src/haproxy.c b/src/haproxy.c
index 8525593e2..810d59838 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -3030,7 +3030,7 @@ int main(int argc, char **argv)
 		int          i;
 		sigset_t     blocked_sig, old_sig;
 
-		THREAD_SYNC_INIT(global.nbthread);
+		THREAD_SYNC_INIT();
 
 		/* Init tids array */
 		for (i = 0; i < global.nbthread; i++)
diff --git a/src/hathreads.c b/src/hathreads.c
index c0c5956f5..bfd055aa9 100644
--- a/src/hathreads.c
+++ b/src/hathreads.c
@@ -11,6 +11,7 @@
  */
 
 #include <unistd.h>
+#include <stdlib.h>
 #include <fcntl.h>
 
 #include <common/cfgparse.h>
@@ -31,7 +32,7 @@ void thread_sync_io_handler(int fd)
 static HA_SPINLOCK_T sync_lock;
 static int           threads_sync_pipe[2];
 static unsigned long threads_want_sync = 0;
-volatile unsigned long all_threads_mask  = 0;
+volatile unsigned long all_threads_mask  = 1; // nbthread 1 assumed by default
 
 #if defined(DEBUG_THREAD) || defined(DEBUG_FULL)
 struct lock_stat lock_stats[LOCK_LABELS];
@@ -41,7 +42,7 @@ struct lock_stat lock_stats[LOCK_LABELS];
  * others when a sync is requested. It also initializes the mask of all created
  * threads. It returns 0 on success and -1 if an error occurred.
  */
-int thread_sync_init(int nbthread)
+int thread_sync_init()
 {
 	int rfd;
 
@@ -51,10 +52,6 @@ int thread_sync_init(int nbthread)
 	rfd = threads_sync_pipe[0];
 	fcntl(rfd, F_SETFL, O_NONBLOCK);
 	fd_insert(rfd, thread_sync_io_handler, thread_sync_io_handler, MAX_THREADS_MASK);
-
-	/* we proceed like this to be sure never to overflow the left shift */
-	all_threads_mask = 1UL << (nbthread - 1);
-	all_threads_mask |= all_threads_mask - 1;
 	return 0;
 }
 
@@ -173,4 +170,38 @@ static void __hathreads_init(void)
 	hap_register_build_opts("Built with multi-threading support.", 0);
 }
 
+#endif // USE_THREAD
+
+
+/* Parse the number of threads in argument <arg>, returns it and adjusts a few
+ * internal variables accordingly, or fails and returns zero with an error
+ * reason in <errmsg>. May be called multiple times while parsing.
+ */
+int parse_nbthread(const char *arg, char **err)
+{
+	long nbthread;
+	char *errptr;
+
+	nbthread = strtol(arg, &errptr, 10);
+	if (!*arg || *errptr) {
+		memprintf(err, "passed a missing or unparsable integer value in '%s'", arg);
+		return 0;
+	}
+
+#ifndef USE_THREAD
+	if (nbthread != 1) {
+		memprintf(err, "specified with a value other than 1 while HAProxy is not compiled with threads support. Please check build options for USE_THREAD");
+		return 0;
+	}
+#else
+	if (nbthread < 1 || nbthread > MAX_THREADS) {
+		memprintf(err, "value must be between 1 and %d (was %ld)", MAX_THREADS, nbthread);
+		return 0;
+	}
+
+	/* we proceed like this to be sure never to overflow the left shift */
+	all_threads_mask = 1UL << (nbthread - 1);
+	all_threads_mask |= all_threads_mask - 1;
 #endif
+	return nbthread;
+}