diff --git a/src/core/PseudoRandom.ts b/src/core/PseudoRandom.ts
index 3a88c7a16..be1410870 100644
--- a/src/core/PseudoRandom.ts
+++ b/src/core/PseudoRandom.ts
@@ -16,7 +16,9 @@ export class PseudoRandom {
 
   // Generates a random integer between min (inclusive) and max (exclusive).
   nextInt(min: number, max: number): number {
-    return Math.floor(this.rng() * (max - min)) + min;
+    const lo = Math.floor(min);
+    const hi = Math.floor(max);
+    return Math.floor(this.rng() * (hi - lo)) + lo;
   }
 
   // Generates a random float between min (inclusive) and max (exclusive).
diff --git a/src/core/execution/WarshipExecution.ts b/src/core/execution/WarshipExecution.ts
index 527056df6..121e7f2d7 100644
--- a/src/core/execution/WarshipExecution.ts
+++ b/src/core/execution/WarshipExecution.ts
@@ -242,12 +242,17 @@ export class WarshipExecution implements Execution {
     // Get warship's water component for connectivity check
     const warshipComponent = this.mg.getWaterComponent(this.warship.tile());
 
+    const patrolTile = this.warship.patrolTile();
+    if (patrolTile === undefined) {
+      return undefined;
+    }
+
     while (expandCount < 3) {
       const x =
-        this.mg.x(this.warship.patrolTile()!) +
+        this.mg.x(patrolTile) +
         this.random.nextInt(-warshipPatrolRange / 2, warshipPatrolRange / 2);
       const y =
-        this.mg.y(this.warship.patrolTile()!) +
+        this.mg.y(patrolTile) +
         this.random.nextInt(-warshipPatrolRange / 2, warshipPatrolRange / 2);
       if (!this.mg.isValidCoord(x, y)) {
         continue;
diff --git a/src/core/game/GameMap.ts b/src/core/game/GameMap.ts
index b885a9403..592d02ca4 100644
--- a/src/core/game/GameMap.ts
+++ b/src/core/game/GameMap.ts
@@ -167,7 +167,14 @@ export class GameMapImpl implements GameMap {
   }
 
   isValidCoord(x: number, y: number): boolean {
-    return x >= 0 && x < this.width_ && y >= 0 && y < this.height_;
+    return (
+      Number.isInteger(x) &&
+      Number.isInteger(y) &&
+      x >= 0 &&
+      x < this.width_ &&
+      y >= 0 &&
+      y < this.height_
+    );
   }
 
   // Terrain getters (immutable)