diff --git a/src/core/game/Game.ts b/src/core/game/Game.ts
index fda7d78c4..861fc70e5 100644
--- a/src/core/game/Game.ts
+++ b/src/core/game/Game.ts
@@ -324,6 +324,8 @@ export class PlayerInfo {
 }
 
 export interface Unit {
+  hash(): number;
+
   // Common properties.
   id(): number;
   type(): UnitType;
diff --git a/src/core/game/PlayerImpl.ts b/src/core/game/PlayerImpl.ts
index 989afc109..03d0a3b27 100644
--- a/src/core/game/PlayerImpl.ts
+++ b/src/core/game/PlayerImpl.ts
@@ -925,7 +925,7 @@ export class PlayerImpl implements Player {
   hash(): number {
     return (
       simpleHash(this.id()) * (this.population() + this.numTilesOwned()) +
-      this._units.reduce((acc, unit) => acc + (unit as UnitImpl).hash(), 0)
+      this._units.reduce((acc, unit) => acc + unit.hash(), 0)
     );
   }
   toString(): string {