diff --git a/core/src/com/unciv/logic/automation/civilization/NextTurnAutomation.kt b/core/src/com/unciv/logic/automation/civilization/NextTurnAutomation.kt
index 3a155dab85..da2da49cab 100644
--- a/core/src/com/unciv/logic/automation/civilization/NextTurnAutomation.kt
+++ b/core/src/com/unciv/logic/automation/civilization/NextTurnAutomation.kt
@@ -293,7 +293,9 @@ object NextTurnAutomation {
     }
 
     private fun useGoldForCityStates(civ: Civilization) {
-        val knownCityStates = civ.getKnownCivs().filter { it.isCityState() }
+        // RARE EDGE CASE: If you ally with a city-state, you may reveal more map that includes ANOTHER civ!
+        // So if we don't lock this list, we may later discover that there are more known civs, concurrent modification exception!
+        val knownCityStates = civ.getKnownCivs().filter { it.isCityState() }.toList()
 
         // canBeMarriedBy checks actual cost, but it can't be below 500*speedmodifier, and the later check is expensive
         if (civ.gold >= 330 && civ.getHappiness() > 0 && civ.hasUnique(UniqueType.CityStateCanBeBoughtForGold)) {