diff --git a/backend/src/routes/resources/main.ts b/backend/src/routes/resources/main.ts
index 547cb64..72d617f 100644
--- a/backend/src/routes/resources/main.ts
+++ b/backend/src/routes/resources/main.ts
@@ -230,11 +230,37 @@ function isDateLikeField(key: string) {
     return /(^|_|-)date($|_|-)/i.test(key)
 }
 
+function isDateValue(value: any) {
+    if (value instanceof Date) return !Number.isNaN(value.getTime())
+    if (typeof value !== "string") return false
+    const normalized = value.trim()
+    if (/^\d+$/.test(normalized)) return false
+    return !Number.isNaN(new Date(normalized).getTime())
+}
+
 function normalizeCreatedDocumentPayload(payload: Record<string, any>) {
+    const numberRelationFields = [
+        "customer",
+        "contact",
+        "project",
+        "createddocument",
+        "letterhead",
+        "plant",
+        "contract",
+        "outgoingsepamandate",
+    ]
+
+    for (const field of numberRelationFields) {
+        const value = payload[field]
+        if (value instanceof Date || (typeof value === "string" && isDateValue(value))) {
+            payload[field] = null
+        }
+    }
+
     const serialexecution = payload.serialexecution
     if (serialexecution === undefined || serialexecution === null || serialexecution === "") return payload
 
-    if (serialexecution instanceof Date) {
+    if (isDateValue(serialexecution)) {
         payload.serialexecution = null
         return payload
     }
@@ -242,7 +268,7 @@ function normalizeCreatedDocumentPayload(payload: Record<string, any>) {
     if (typeof serialexecution === "string") {
         const normalized = serialexecution.trim()
         const isUuid = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i.test(normalized)
-        if (!isUuid && !Number.isNaN(new Date(normalized).getTime())) {
+        if (!isUuid && isDateValue(normalized)) {
             payload.serialexecution = null
             return payload
         }