diff --git a/src/main/kotlin/io/gitlab/jfronny/sdom/model/Contest.kt b/src/main/kotlin/io/gitlab/jfronny/sdom/model/Contest.kt
index 89e3075..8cfa608 100644
--- a/src/main/kotlin/io/gitlab/jfronny/sdom/model/Contest.kt
+++ b/src/main/kotlin/io/gitlab/jfronny/sdom/model/Contest.kt
@@ -15,7 +15,7 @@ data class Contest(
     //not needed here val banner: List<ImageFile>,
     //not needed here val problemset: List<FileWithName>,
     val id: String,
-    @SerialName("external_id") val externalId: String,
+    @SerialName("external_id") val externalId: String?,
     val name: String,
     val shortname: String,
     @SerialName("allow_submit") val allowSubmit: Boolean,
diff --git a/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDJudgementType.kt b/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDJudgementType.kt
index 91403f2..d223dda 100644
--- a/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDJudgementType.kt
+++ b/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDJudgementType.kt
@@ -1,5 +1,8 @@
 package io.gitlab.jfronny.sdom.model
 
+import kotlinx.serialization.Serializable
+
+@Serializable
 data class SDJudgementType(
     val id: String,
     val name: String,
diff --git a/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDSubmission.kt b/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDSubmission.kt
index caa17a1..5869971 100644
--- a/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDSubmission.kt
+++ b/src/main/kotlin/io/gitlab/jfronny/sdom/model/SDSubmission.kt
@@ -1,7 +1,9 @@
 package io.gitlab.jfronny.sdom.model
 
 import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
 
+@Serializable
 data class SDSubmission(
     @SerialName("language_id") val languageId: String,
     val time: String,