diff --git a/src/modbus-data.c b/src/modbus-data.c
index 75703bd..0f1453f 100644
--- a/src/modbus-data.c
+++ b/src/modbus-data.c
@@ -56,6 +56,7 @@ uint8_t modbus_get_byte_from_bits(const uint8_t *src, int address,
     uint8_t value = 0;
 
     if (nb_bits > 8) {
+        /* Assert is ignored if NDEBUG is set */
         assert(nb_bits < 8);
         nb_bits = 8;
     }
diff --git a/src/modbus-rtu.c b/src/modbus-rtu.c
index 98c6652..dac16a3 100644
--- a/src/modbus-rtu.c
+++ b/src/modbus-rtu.c
@@ -21,6 +21,7 @@
 #include <fcntl.h>
 #include <string.h>
 #include <unistd.h>
+#include <assert.h>
 
 #include "modbus-private.h"
 
@@ -87,6 +88,8 @@ static const uint8_t table_crc_lo[] = {
     0x43, 0x83, 0x41, 0x81, 0x80, 0x40
 };
 
+/* Define the slave ID of the remote device to talk in master mode or set the
+ * internal slave ID in slave mode */
 static int _modbus_set_slave(modbus_t *ctx, int slave)
 {
     if (slave >= 1 && slave <= 247) {
@@ -104,6 +107,7 @@ static int _modbus_rtu_build_request_basis(modbus_t *ctx, int function,
                                            int addr, int nb,
                                            uint8_t *req)
 {
+    assert(ctx->slave != -1);
     req[0] = ctx->slave;
     req[1] = function;
     req[2] = addr >> 8;
@@ -117,6 +121,8 @@ static int _modbus_rtu_build_request_basis(modbus_t *ctx, int function,
 /* Builds a RTU response header */
 static int _modbus_rtu_build_response_basis(sft_t *sft, uint8_t *rsp)
 {
+    /* In this case, the slave is certainly valid because a check is already
+     * done in _modbus_rtu_listen */
     rsp[0] = sft->slave;
     rsp[1] = sft->function;
 
@@ -712,6 +718,14 @@ int _modbus_rtu_listen(modbus_t *ctx, int nb_connection)
         fprintf(stderr, "Not implemented");
     }
 
+    if (ctx->slave == -1) {
+        if (ctx->debug) {
+            fprintf(stderr, "The slave ID is not set (you must call modbus_set_slave() first)\n");
+        }
+        errno = EINVAL;
+        return -1;
+    }
+
     errno = EINVAL;
     return -1;
 }
@@ -832,23 +846,18 @@ const modbus_backend_t _modbus_rtu_backend = {
    - parity: 'N' stands for None, 'E' for Even and 'O' for odd
    - data_bits: 5, 6, 7, 8
    - stop_bits: 1, 2
-   - slave: slave number of the caller
 */
 modbus_t* modbus_new_rtu(const char *device,
                          int baud, char parity, int data_bit,
-                         int stop_bit, int slave)
+                         int stop_bit)
 {
     modbus_t *ctx;
     modbus_rtu_t *ctx_rtu;
 
     ctx = (modbus_t *) malloc(sizeof(modbus_t));
     _modbus_init_common(ctx);
-    if (_modbus_set_slave(ctx, slave) == -1) {
-        return NULL;
-    }
 
     ctx->backend = &_modbus_rtu_backend;
-
     ctx->backend_data = (modbus_rtu_t *) malloc(sizeof(modbus_rtu_t));
     ctx_rtu = (modbus_rtu_t *)ctx->backend_data;
 #if defined(OpenBSD)
diff --git a/src/modbus-rtu.h b/src/modbus-rtu.h
index e312606..edfcda2 100644
--- a/src/modbus-rtu.h
+++ b/src/modbus-rtu.h
@@ -26,6 +26,6 @@
 #define MODBUS_RTU_MAX_ADU_LENGTH  256
 
 modbus_t* modbus_new_rtu(const char *device, int baud, char parity,
-                         int data_bit, int stop_bit, int slave);
+                         int data_bit, int stop_bit);
 
 #endif /* _MODBUS_RTU_H_ */
diff --git a/src/modbus.c b/src/modbus.c
index 2c437f0..1cd9aed 100644
--- a/src/modbus.c
+++ b/src/modbus.c
@@ -1225,14 +1225,17 @@ int modbus_report_slave_id(modbus_t *ctx, uint8_t *data_dest)
 
 void _modbus_init_common(modbus_t *ctx)
 {
+    /* Slave is initialized to -1 */
+    ctx->slave = -1;
+
+    ctx->debug = FALSE;
+    ctx->error_recovery = FALSE;
+
     ctx->timeout_begin.tv_sec = 0;
     ctx->timeout_begin.tv_usec = _TIME_OUT_BEGIN_OF_TRAME;
 
     ctx->timeout_end.tv_sec = 0;
     ctx->timeout_end.tv_usec = _TIME_OUT_END_OF_TRAME;
-
-    ctx->error_recovery = FALSE;
-    ctx->debug = FALSE;
 }
 
 /* Define the slave number */
diff --git a/tests/random-test-client.c b/tests/random-test-client.c
index f6f6cef..5ff1d91 100644
--- a/tests/random-test-client.c
+++ b/tests/random-test-client.c
@@ -37,7 +37,6 @@
    range defined by the following defines.
 */
 #define LOOP             1
-#define MY_ID            1
 #define SERVER_ID       17
 #define ADDRESS_START    0
 #define ADDRESS_END     99
@@ -59,14 +58,16 @@ int main(void)
     uint16_t *tab_rw_rq_registers;
     uint16_t *tab_rp_registers;
 
-    /*
-    ctx = modbus_new_rtu("/dev/ttyS0", 19200, 'N', 8, 1, MY_ID);
+    /* RTU */
+/*
+    ctx = modbus_new_rtu("/dev/ttyUSB0", 19200, 'N', 8, 1);
     modbus_set_slave(ctx, SERVER_ID);
-    */
+*/
 
     /* TCP */
     ctx = modbus_new_tcp("127.0.0.1", 1502);
     modbus_set_debug(ctx, TRUE);
+
     if (modbus_connect(ctx) == -1) {
         fprintf(stderr, "Connection failed: %s\n",
                 modbus_strerror(errno));
diff --git a/tests/unit-test-client.c b/tests/unit-test-client.c
index d9fbe72..fb853fa 100644
--- a/tests/unit-test-client.c
+++ b/tests/unit-test-client.c
@@ -41,7 +41,7 @@ int main(void)
     struct timeval timeout_begin_new;
 
     /*
-      ctx = modbus_new_rtu("/dev/ttyS0", 19200, 'N', 8, 1, CLIENT_ID);
+      ctx = modbus_new_rtu("/dev/ttyUSB0", 19200, 'N', 8, 1);
       modbus_set_slave(ctx, SERVER_ID);
       is_mode_rtu = TRUE;
     */
diff --git a/tests/unit-test.h b/tests/unit-test.h
index bf0402a..a4efd2c 100644
--- a/tests/unit-test.h
+++ b/tests/unit-test.h
@@ -25,7 +25,6 @@
 #include <stdint.h>
 #endif
 
-#define CLIENT_ID 15
 #define SERVER_ID 17
 
 const uint16_t UT_BITS_ADDRESS = 0x13;