diff --git a/client/src/pages/vehicles.tsx b/client/src/pages/vehicles.tsx
index 82f35f0..79ed898 100644
--- a/client/src/pages/vehicles.tsx
+++ b/client/src/pages/vehicles.tsx
@@ -143,7 +143,11 @@ export default function Vehicles() {
 
   const createVehicleMutation = useMutation({
     mutationFn: async (data: VehicleForm) => {
-      return apiRequest("POST", "/api/vehicles", data);
+      const payload = {
+        ...data,
+        assignedGuardId: data.assignedGuardId === "unassigned" ? null : data.assignedGuardId,
+      };
+      return apiRequest("POST", "/api/vehicles", payload);
     },
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ["/api/vehicles"] });
@@ -518,7 +522,7 @@ export default function Vehicles() {
                           </SelectTrigger>
                         </FormControl>
                         <SelectContent>
-                          <SelectItem value="">Nessuno</SelectItem>
+                          <SelectItem value="unassigned">Nessuno</SelectItem>
                           {guards?.map(guard => (
                             <SelectItem key={guard.id} value={guard.id}>
                               {guard.badgeNumber}
@@ -745,7 +749,7 @@ export default function Vehicles() {
                           </SelectTrigger>
                         </FormControl>
                         <SelectContent>
-                          <SelectItem value="">Nessuno</SelectItem>
+                          <SelectItem value="unassigned">Nessuno</SelectItem>
                           {guards?.map(guard => (
                             <SelectItem key={guard.id} value={guard.id}>
                               {guard.badgeNumber}