diff --git a/js/data.js b/js/data.js
index 4deced3..3b8cb63 100644
--- a/js/data.js
+++ b/js/data.js
@@ -1,9 +1,22 @@
 export function loadFromJSON(json) {
-  const parsed = JSON.parse(json);
+  let parsed;
+  try {
+    parsed = JSON.parse(json);
+  } catch (e) {
+    throw new Error('Ungültige JSON-Datei: ' + e.message);
+  }
+  if (typeof parsed !== 'object' || parsed === null) {
+    throw new Error('Ungültiges Dateiformat: Objekt erwartet');
+  }
   return {
-    employees: parsed.employees ?? [],
-    calendar: parsed.calendar ?? { holidays: [], companyClosures: [] },
-    history: parsed.history ?? []
+    employees: Array.isArray(parsed.employees) ? parsed.employees : [],
+    calendar: (parsed.calendar && typeof parsed.calendar === 'object')
+      ? {
+          holidays: Array.isArray(parsed.calendar.holidays) ? parsed.calendar.holidays : [],
+          companyClosures: Array.isArray(parsed.calendar.companyClosures) ? parsed.calendar.companyClosures : []
+        }
+      : { holidays: [], companyClosures: [] },
+    history: Array.isArray(parsed.history) ? parsed.history : []
   };
 }
 
@@ -16,9 +29,11 @@ export function saveToJSON(state) {
 }
 
 export function createEmployee(name) {
+  const trimmed = (name ?? '').trim();
+  if (!trimmed) throw new Error('Mitarbeitername darf nicht leer sein');
   return {
     id: crypto.randomUUID(),
-    name: name.trim(),
+    name: trimmed,
     constraints: {
       neverDays: [],
       lowPriorityDays: [],
@@ -48,6 +63,7 @@ export function toggleClosure(state, isoDate) {
 
 // Adds entries to history, replacing any existing entries for the same dates
 export function addHistoryEntries(state, entries) {
+  if (!entries?.length) return;
   const dates = new Set(entries.map(e => e.date));
   state.history = state.history.filter(h => !dates.has(h.date));
   state.history.push(...entries);