diff --git a/demo/WebAuthn.Net.Demo.Mvc/Controllers/PasswordlessController.cs b/demo/WebAuthn.Net.Demo.Mvc/Controllers/PasswordlessController.cs
index ce3dd24..0bf6739 100644
--- a/demo/WebAuthn.Net.Demo.Mvc/Controllers/PasswordlessController.cs
+++ b/demo/WebAuthn.Net.Demo.Mvc/Controllers/PasswordlessController.cs
@@ -95,7 +95,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (authenticationCeremonyId is null)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "Authentication ceremony not found");
+            return BadRequest(ModelState);
         }
 
         var result = await _authenticationCeremonyService.CompleteCeremonyAsync(
@@ -106,7 +107,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (result.HasError)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "The authentication ceremony completed with an error");
+            return BadRequest(ModelState);
         }
 
         var applicationUser = await _userService.FindAsync(
@@ -115,7 +117,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (applicationUser is null)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "User not found");
+            return BadRequest(ModelState);
         }
 
         var claims = new List<Claim>
diff --git a/demo/WebAuthn.Net.Demo.Mvc/Controllers/RegistrationController.cs b/demo/WebAuthn.Net.Demo.Mvc/Controllers/RegistrationController.cs
index bbc2b64..f2a44ce 100644
--- a/demo/WebAuthn.Net.Demo.Mvc/Controllers/RegistrationController.cs
+++ b/demo/WebAuthn.Net.Demo.Mvc/Controllers/RegistrationController.cs
@@ -76,7 +76,8 @@ public async Task<IActionResult> CompleteRegistration(
             cancellationToken);
         if (registrationCeremonyId is null)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "Registration ceremony not found");
+            return BadRequest(ModelState);
         }
 
         var result = await _registrationCeremonyService.CompleteCeremonyAsync(
@@ -88,7 +89,8 @@ public async Task<IActionResult> CompleteRegistration(
             cancellationToken);
         if (result.HasError)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "The registration ceremony completed with an error");
+            return BadRequest(ModelState);
         }
 
         await _registrationCeremonyHandleService.DeleteAsync(HttpContext, cancellationToken);
diff --git a/demo/WebAuthn.Net.Demo.Mvc/Controllers/UsernamelessController.cs b/demo/WebAuthn.Net.Demo.Mvc/Controllers/UsernamelessController.cs
index dfc4d31..33555ab 100644
--- a/demo/WebAuthn.Net.Demo.Mvc/Controllers/UsernamelessController.cs
+++ b/demo/WebAuthn.Net.Demo.Mvc/Controllers/UsernamelessController.cs
@@ -84,7 +84,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (authenticationCeremonyId is null)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "Authentication ceremony not found");
+            return BadRequest(ModelState);
         }
 
         var result = await _authenticationCeremonyService.CompleteCeremonyAsync(
@@ -95,7 +96,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (result.HasError)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "The authentication ceremony completed with an error");
+            return BadRequest(ModelState);
         }
 
         var applicationUser = await _userService.FindAsync(
@@ -104,7 +106,8 @@ public async Task<IActionResult> CompleteAuthentication(
             cancellationToken);
         if (applicationUser is null)
         {
-            return BadRequest();
+            ModelState.AddModelError("", "User not found");
+            return BadRequest(ModelState);
         }
 
         var claims = new List<Claim>