diff --git a/components/freemodbus/port/porttimer.c b/components/freemodbus/port/porttimer.c
index f63377a..c68e171 100644
--- a/components/freemodbus/port/porttimer.c
+++ b/components/freemodbus/port/porttimer.c
@@ -121,7 +121,7 @@ BOOL xMBPortTimersInit(USHORT usTim1Timerout50us)
                     "failure to set alarm failure, timer_set_alarm_value() returned (0x%x).",
                     (uint32_t)xErr);
     // Register ISR for timer
-    xErr = timer_isr_register(usTimerGroupIndex, usTimerIndex, vTimerGroupIsr, NULL, ESP_INTR_FLAG_IRAM, NULL);
+    xErr = timer_isr_register(usTimerGroupIndex, usTimerIndex, vTimerGroupIsr, (void*)(uint32_t)usTimerIndex, ESP_INTR_FLAG_IRAM, NULL);
     MB_PORT_CHECK((xErr == ESP_OK), FALSE,
                     "timer set value failure, timer_isr_register() returned (0x%x).",
                     (uint32_t)xErr);
diff --git a/components/freemodbus/port/porttimer_m.c b/components/freemodbus/port/porttimer_m.c
index 1742623..55c561c 100644
--- a/components/freemodbus/port/porttimer_m.c
+++ b/components/freemodbus/port/porttimer_m.c
@@ -128,7 +128,7 @@ BOOL xMBMasterPortTimersInit(USHORT usTimeOut50us)
                     (uint32_t)xErr);
     // Register ISR for timer
     xErr = timer_isr_register(usTimerGroupIndex, usTimerIndex,
-                                vTimerGroupIsr, NULL, ESP_INTR_FLAG_IRAM, NULL);
+                                vTimerGroupIsr, (void*)(uint32_t)usTimerIndex, ESP_INTR_FLAG_IRAM, NULL);
     MB_PORT_CHECK((xErr == ESP_OK), FALSE,
                     "timer set value failure, timer_isr_register() returned (0x%x).",
                     (uint32_t)xErr);