diff --git a/core/src/com/unciv/logic/GameInfo.kt b/core/src/com/unciv/logic/GameInfo.kt
index 0c739457ad..54ef8fbc43 100644
--- a/core/src/com/unciv/logic/GameInfo.kt
+++ b/core/src/com/unciv/logic/GameInfo.kt
@@ -56,6 +56,13 @@ import java.util.UUID
  * When you change the structure of any class with this interface in a way which makes it impossible
  * to load the new saves from an older game version, increment [CURRENT_COMPATIBILITY_NUMBER]! And don't forget
  * to add backwards compatibility for the previous format.
+ *
+ * Reminder: In all subclasse, do use only actual Collection types, not abstractions like
+ * `= mutableSetOf<Something>()`. That would make the reflection type of the field an interface, which
+ * hides the actual implementation from Gdx Json, so it will not try to call a no-args constructor but
+ * will instead deserialize a List in the jsonData.isArray() -> isAssignableFrom(Collection) branch of readValue:
+ * https://github.com/libgdx/libgdx/blob/75612dae1eeddc9611ed62366858ff1d0ac7898b/gdx/src/com/badlogic/gdx/utils/Json.java#L1111
+ * .. which will crash later (when readFields actually assigns it) unless empty.
  */
 interface IsPartOfGameInfoSerialization
 
diff --git a/core/src/com/unciv/logic/civilization/managers/EspionageManager.kt b/core/src/com/unciv/logic/civilization/managers/EspionageManager.kt
index 9c5946e7b8..996f26df73 100644
--- a/core/src/com/unciv/logic/civilization/managers/EspionageManager.kt
+++ b/core/src/com/unciv/logic/civilization/managers/EspionageManager.kt
@@ -8,15 +8,15 @@ import com.unciv.models.Spy
 
 class EspionageManager : IsPartOfGameInfoSerialization {
 
-    var spyList = mutableListOf<Spy>()
-    val erasSpyEarnedFor = mutableSetOf<String>()
+    var spyList = ArrayList<Spy>()
+    val erasSpyEarnedFor = LinkedHashSet<String>()
 
     @Transient
     lateinit var civInfo: Civilization
 
     fun clone(): EspionageManager {
         val toReturn = EspionageManager()
-        toReturn.spyList.addAll(spyList.map { it.clone() })
+        spyList.mapTo(toReturn.spyList) { it.clone() }
         toReturn.erasSpyEarnedFor.addAll(erasSpyEarnedFor)
         return toReturn
     }