diff --git a/modules/openapi-generator/src/main/resources/csharp/auth/TokenResponse.mustache b/modules/openapi-generator/src/main/resources/csharp/auth/TokenResponse.mustache
index becd82d87a1..7a72e04c12c 100644
--- a/modules/openapi-generator/src/main/resources/csharp/auth/TokenResponse.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp/auth/TokenResponse.mustache
@@ -19,6 +19,6 @@ namespace {{packageName}}.Client.Auth
         [JsonProperty("refresh_token")]
         public string{{nrt?}} RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index ab1cf58090d..5ff4349979f 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/MultipleFrameworks/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index d3703f73dd7..95e03c36e10 100644
--- a/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index d3703f73dd7..95e03c36e10 100644
--- a/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/net4.8/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index 9b9fc94bbf8..da4f4499b1c 100644
--- a/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/EnumMappings/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string? RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index 9b9fc94bbf8..da4f4499b1c 100644
--- a/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/net7/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string? RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index d3703f73dd7..95e03c36e10 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
index d3703f73dd7..95e03c36e10 100644
--- a/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
+++ b/samples/client/petstore/csharp/restsharp/standard2.0/Petstore/src/Org.OpenAPITools/Client/Auth/TokenResponse.cs
@@ -27,6 +27,6 @@ namespace Org.OpenAPITools.Client.Auth
         [JsonProperty("refresh_token")]
         public string RefreshToken { get; set; }
 
-        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created.AddSeconds(ExpiresIn.Value);
+        public DateTime? ExpiresAt => ExpiresIn == null ? null : Created?.AddSeconds(ExpiresIn.Value);
     }
 }
\ No newline at end of file