diff --git a/src/schdispatch_gui.cpp b/src/schdispatch_gui.cpp
index 9f58e287dd..71c2926cb0 100644
--- a/src/schdispatch_gui.cpp
+++ b/src/schdispatch_gui.cpp
@@ -473,9 +473,10 @@ struct SchdispatchWindow : Window {
 			default: NOT_REACHED();
 
 			case WID_SCHDISPATCH_ADD: {
-				int32 val = StrEmpty(str) ? -1 : strtoul(str, NULL, 10);
+				char *end;
+				int32 val = StrEmpty(str) ? -1 : strtoul(str, &end, 10);
 
-				if (val >= 0) {
+				if (val >= 0 && end && *end == 0) {
 					uint minutes = (val % 100) % 60;
 					uint hours = (val / 100) % 24;
 					val = MINUTES_DATE(MINUTES_DAY(CURRENT_MINUTE), hours, minutes);
@@ -487,9 +488,10 @@ struct SchdispatchWindow : Window {
 			}
 
 			case WID_SCHDISPATCH_SET_START_DATE: {
-				int32 val = StrEmpty(str) ? -1 : strtoul(str, NULL, 10);
+				char *end;
+				int32 val = StrEmpty(str) ? -1 : strtoul(str, &end, 10);
 
-				if (val >= 0) {
+				if (val >= 0 && end && *end == 0) {
 					uint minutes = (val % 100) % 60;
 					uint hours = (val / 100) % 24;
 					val = MINUTES_DATE(MINUTES_DAY(CURRENT_MINUTE), hours, minutes);
@@ -516,9 +518,10 @@ struct SchdispatchWindow : Window {
 			}
 
 			case WID_SCHDISPATCH_SET_DELAY: {
-				int32 val = StrEmpty(str) ? -1 : strtoul(str, NULL, 10);
+				char *end;
+				int32 val = StrEmpty(str) ? -1 : strtoul(str, &end, 10);
 
-				if (val >= 0) {
+				if (val >= 0 && end && *end == 0) {
 					if (_settings_client.gui.time_in_minutes) {
 						val *= _settings_client.gui.ticks_per_minute;
 					} else {