Index: vendor/magento/module-quote-graph-ql/Model/Resolver/MergeCarts.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/vendor/magento/module-quote-graph-ql/Model/Resolver/MergeCarts.php b/vendor/magento/module-quote-graph-ql/Model/Resolver/MergeCarts.php
--- a/vendor/magento/module-quote-graph-ql/Model/Resolver/MergeCarts.php	
+++ b/vendor/magento/module-quote-graph-ql/Model/Resolver/MergeCarts.php	(date 1726475385659)
@@ -149,27 +149,31 @@
 
         $storeId = (int)$context->getExtensionAttributes()->getStore()->getId();
         // passing customerId as null enforces source cart should always be a guestcart
-        $guestCart = $this->getCartForUser->execute(
-            $guestMaskedCartId,
-            null,
-            $storeId
-        );
-        $customerCart = $this->getCartForUser->execute(
-            $customerMaskedCartId,
-            $currentUserId,
-            $storeId
-        );
-        if ($this->cartQuantityValidator->validateFinalCartQuantities($customerCart, $guestCart)) {
-            $guestCart = $this->getCartForUser->execute(
-                $guestMaskedCartId,
-                null,
-                $storeId
-            );
-        }
-        $customerCart->merge($guestCart);
-        $guestCart->setIsActive(false);
-        $this->cartRepository->save($customerCart);
-        $this->cartRepository->save($guestCart);
+        try {
+            $guestCart = $this->getCartForUser->execute(
+                $guestMaskedCartId,
+                null,
+                $storeId
+            );
+            $customerCart = $this->getCartForUser->execute(
+                $customerMaskedCartId,
+                $currentUserId,
+                $storeId
+            );
+            if ($this->cartQuantityValidator->validateFinalCartQuantities($customerCart, $guestCart)) {
+                $guestCart = $this->getCartForUser->execute(
+                    $guestMaskedCartId,
+                    null,
+                    $storeId
+                );
+            }
+            $customerCart->merge($guestCart);
+            $guestCart->setIsActive(false);
+            $this->cartRepository->save($guestCart);
+        } catch (\Exception $exception) {
+            $customerCart = $this->getCartForUser->execute($customerMaskedCartId, $currentUserId, $storeId);
+        }
+        $this->cartRepository->save($customerCart);
         return [
             'model' => $customerCart,
         ];