diff --git a/common/src/algorithms/movementDirection.ts b/common/src/algorithms/movementDirection.ts
index 873b183..29f34ae 100644
--- a/common/src/algorithms/movementDirection.ts
+++ b/common/src/algorithms/movementDirection.ts
@@ -10,9 +10,20 @@
  *	the target ship
  */
 export function countShips(board: any[], shipLocation: number) {
+	if (shipLocation < 0) {
+		throw new Error("shipLocation must be >= 0");
+	};
+
+	// Ensure that when shipLocation we don't assume all ships are to the left
+	// of it due to negative indexing in .slice() being supported
+	let left = 0;
+	if (shipLocation > 0) {
+		left = board.slice(0, shipLocation - 1).filter(x => x != null).length;
+	};
+
 	return {
-		left: board.slice(1, shipLocation - 1).filter(x => x != null).length,
-		right: board.slice(shipLocation + 1).filter(x => x != null).length,
+		left,
+		right: board.slice(shipLocation + 1).filter(x => x != null).length
 	};
 };