diff --git a/TaxBaik.Web/Components/Admin/Pages/Contracts.razor b/TaxBaik.Web/Components/Admin/Pages/Contracts.razor
index 4b70578..a31ec56 100644
--- a/TaxBaik.Web/Components/Admin/Pages/Contracts.razor
+++ b/TaxBaik.Web/Components/Admin/Pages/Contracts.razor
@@ -107,10 +107,10 @@
     </TitleContent>
     <DialogContent>
         <MudForm @ref="form">
-            <MudSelect T="int" @bind-Value="contractForm.ClientId" Label="고객" Required="true" Variant="Variant.Outlined" FullWidth="true" Class="mb-4">
+            <MudSelect T="int?" @bind-Value="contractForm.ClientId" Label="고객" Required="true" Variant="Variant.Outlined" FullWidth="true" Class="mb-4" RequiredError="고객을 선택하세요.">
                 @foreach (var client in clients)
                 {
-                    <MudSelectItem Value="@client.Id">@client.CompanyName</MudSelectItem>
+                    <MudSelectItem Value="@((int?)client.Id)">@(string.IsNullOrEmpty(client.CompanyName) ? client.Name : client.CompanyName)</MudSelectItem>
                 }
             </MudSelect>
             <MudTextField T="string" @bind-Value="contractForm.ContractNumber" Label="계약번호" Variant="Variant.Outlined" FullWidth="true" Class="mb-4" Required="true" />
@@ -180,7 +180,7 @@
     {
         contractForm = new ContractForm
         {
-            ClientId = clients.FirstOrDefault()?.Id ?? 0,
+            ClientId = clients.FirstOrDefault()?.Id,
             StartDate = DateTime.Today
         };
         isDialogOpen = true;
@@ -200,8 +200,9 @@
 
         try
         {
+            if (contractForm.ClientId == null) return;
             var newId = await ContractClient.CreateAsync(
-                contractForm.ClientId,
+                contractForm.ClientId.Value,
                 contractForm.ContractNumber,
                 contractForm.ServiceType,
                 contractForm.StartDate ?? DateTime.Now,
@@ -254,7 +255,7 @@
 
     private class ContractForm
     {
-        public int ClientId { get; set; }
+        public int? ClientId { get; set; }
         public string ContractNumber { get; set; } = "";
         public string ServiceType { get; set; } = "";
         public DateTime? StartDate { get; set; }
diff --git a/TaxBaik.Web/Components/Admin/Pages/TaxFilingSchedules.razor b/TaxBaik.Web/Components/Admin/Pages/TaxFilingSchedules.razor
index 14ce07b..51e45ca 100644
--- a/TaxBaik.Web/Components/Admin/Pages/TaxFilingSchedules.razor
+++ b/TaxBaik.Web/Components/Admin/Pages/TaxFilingSchedules.razor
@@ -117,16 +117,17 @@
     </TitleContent>
     <DialogContent>
         <MudForm @ref="form">
-            <MudSelect T="int"
+            <MudSelect T="int?"
                        @bind-Value="scheduleForm.ClientId"
                        Label="고객"
                        Required="true"
                        Variant="Variant.Outlined"
                        FullWidth="true"
-                       Class="mb-4">
+                       Class="mb-4"
+                       RequiredError="고객을 선택하세요.">
                 @foreach (var client in clients)
                 {
-                    <MudSelectItem Value="@client.Id">@client.CompanyName</MudSelectItem>
+                    <MudSelectItem Value="@((int?)client.Id)">@(string.IsNullOrEmpty(client.CompanyName) ? client.Name : client.CompanyName)</MudSelectItem>
                 }
             </MudSelect>
             <MudSelect T="string" @bind-Value="scheduleForm.FilingType" Label="신고 유형" Variant="Variant.Outlined" FullWidth="true" Class="mb-4" Required="true">
@@ -197,7 +198,7 @@
         {
             FilingYear = DateTime.Now.Year,
             DueDate = DateTime.Today,
-            ClientId = clients.FirstOrDefault()?.Id ?? 0
+            ClientId = clients.FirstOrDefault()?.Id
         };
         isDialogOpen = true;
     }
@@ -216,8 +217,9 @@
 
         try
         {
+            if (scheduleForm.ClientId == null) return;
             var newId = await TaxFilingClient.CreateAsync(
-                scheduleForm.ClientId,
+                scheduleForm.ClientId.Value,
                 scheduleForm.FilingType,
                 scheduleForm.DueDate ?? DateTime.Today,
                 scheduleForm.FilingYear);
diff --git a/TaxBaik.Web/Components/Admin/Pages/TaxProfiles.razor b/TaxBaik.Web/Components/Admin/Pages/TaxProfiles.razor
index 4b29aaa..4098001 100644
--- a/TaxBaik.Web/Components/Admin/Pages/TaxProfiles.razor
+++ b/TaxBaik.Web/Components/Admin/Pages/TaxProfiles.razor
@@ -84,10 +84,10 @@ else
     </TitleContent>
     <DialogContent>
         <MudForm @ref="form">
-            <MudSelect T="int" @bind-Value="profileForm.ClientId" Label="고객" Required="true" Variant="Variant.Outlined" FullWidth="true" Class="mb-4">
+            <MudSelect T="int?" @bind-Value="profileForm.ClientId" Label="고객" Required="true" Variant="Variant.Outlined" FullWidth="true" Class="mb-4" RequiredError="고객을 선택하세요.">
                 @foreach (var client in clients)
                 {
-                    <MudSelectItem Value="@client.Id">@client.CompanyName</MudSelectItem>
+                    <MudSelectItem Value="@((int?)client.Id)">@(string.IsNullOrEmpty(client.CompanyName) ? client.Name : client.CompanyName)</MudSelectItem>
                 }
             </MudSelect>
             <MudSelect T="string" @bind-Value="profileForm.BusinessType" Label="사업 유형" Variant="Variant.Outlined" FullWidth="true" Class="mb-4" Required="true">
@@ -166,7 +166,7 @@ else
         editingProfile = null;
         profileForm = new TaxProfileForm
         {
-            ClientId = clients.FirstOrDefault()?.Id ?? 0,
+            ClientId = clients.FirstOrDefault()?.Id,
             TaxRiskLevel = "normal",
             NextFilingDueDate = DateTime.Today.AddMonths(1)
         };
@@ -202,15 +202,11 @@ else
 
         try
         {
-            if (isEditMode)
+            if (isEditMode && editingProfile != null)
             {
-                await TaxProfileClient.UpdateAsync(
-                    editingProfile!.Id,
-                    profileForm.BusinessType,
-                    null,
-                    profileForm.NextFilingDueDate,
-                    profileForm.TaxRiskLevel);
-                Snackbar.Add("세무 프로필이 업데이트되었습니다.", Severity.Success);
+                await TaxProfileClient.UpdateAsync(editingProfile.Id, profileForm.BusinessType,
+                    null, profileForm.NextFilingDueDate, profileForm.TaxRiskLevel);
+                Snackbar.Add("세무 프로필이 수정되었습니다.", Severity.Success);
             }
             else
             {
diff --git a/tests/e2e/helpers/admin-auth.ts b/tests/e2e/helpers/admin-auth.ts
index 7dd29b4..8a27d90 100644
--- a/tests/e2e/helpers/admin-auth.ts
+++ b/tests/e2e/helpers/admin-auth.ts
@@ -62,6 +62,9 @@ export async function navigateInBlazor(page: Page, targetUrl: string) {
   // Wait until Blazor Server completes connection and hides the loading spinner overlay
   await page.locator('#blazor-loading').waitFor({ state: 'hidden', timeout: 15000 }).catch(() => {});
 
+  // Give the SPA router a brief window to unmount the previous page and mount the loading spinner
+  await page.waitForTimeout(500);
+
   // Also wait for MudBlazor's dynamic loading spinners to disappear (ensuring the grid is interactive)
   const spinner = page.locator('.mud-progress-circular, .mud-progress-linear-bar');
   try {