diff --git a/internal/web_server/handlers/notification_handler.go b/internal/web_server/handlers/notification_handler.go
index 410666f..9f98cc7 100644
--- a/internal/web_server/handlers/notification_handler.go
+++ b/internal/web_server/handlers/notification_handler.go
@@ -6,6 +6,7 @@ import (
 	"bufio"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -16,6 +17,7 @@ import (
 
 	"github.com/gofiber/fiber/v2"
 	"github.com/gorilla/websocket"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/resend/resend-go/v2"
 	"go.uber.org/zap"
 )
@@ -111,7 +113,7 @@ type hijackResponseWriter struct {
 	h    http.Header
 }
 
-func (w *hijackResponseWriter) Header() http.Header        { return w.h }
+func (w *hijackResponseWriter) Header() http.Header         { return w.h }
 func (w *hijackResponseWriter) WriteHeader(statusCode int)  {}
 func (w *hijackResponseWriter) Write(b []byte) (int, error) { return w.conn.Write(b) }
 func (w *hijackResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
@@ -662,6 +664,12 @@ func (h *Handler) RegisterDeviceToken(c *fiber.Ctx) error {
 		)
 		return fiber.NewError(fiber.StatusBadRequest, "Invalid request body")
 	}
+	if valErrs, ok := h.validator.Validate(c, req); !ok {
+		return c.Status(fiber.StatusBadRequest).JSON(domain.ErrorResponse{
+			Message: "Validation failed",
+			Error:   firstValidationError(valErrs),
+		})
+	}
 
 	userID, ok := c.Locals("user_id").(int64)
 	if !ok || userID == 0 {
@@ -673,6 +681,14 @@ func (h *Handler) RegisterDeviceToken(c *fiber.Ctx) error {
 	}
 
 	if err := h.userSvc.RegisterDevice(c.Context(), userID, req.DeviceToken, req.Platform); err != nil {
+		var pgErr *pgconn.PgError
+		if errors.As(err, &pgErr) && pgErr.Code == "23503" && pgErr.ConstraintName == "devices_user_fk" {
+			return c.Status(fiber.StatusBadRequest).JSON(domain.ErrorResponse{
+				Message: "Invalid authenticated user",
+				Error:   "authenticated user does not exist in users table",
+			})
+		}
+
 		h.mongoLoggerSvc.Error("[NotificationHandler.RegisterDeviceToken] Failed to register device token",
 			zap.Int64("userID", userID),
 			zap.String("platform", req.Platform),