diff --git a/Marlin/temperature.cpp b/Marlin/temperature.cpp
index 236ddfc8f..9f99c1368 100644
--- a/Marlin/temperature.cpp
+++ b/Marlin/temperature.cpp
@@ -1013,7 +1013,7 @@ void tp_init() {
   void start_watching_heater(int e) {
     if (degHotend(e) < degTargetHotend(e) - (WATCH_TEMP_INCREASE + TEMP_HYSTERESIS + 1)) {
       watch_target_temp[e] = degHotend(e) + WATCH_TEMP_INCREASE;
-      watch_heater_next_ms[e] = millis() + WATCH_TEMP_PERIOD * 1000;
+      watch_heater_next_ms[e] = millis() + WATCH_TEMP_PERIOD * 1000UL;
     }
     else
       watch_heater_next_ms[e] = 0;