diff --git a/TetraVexKit/TetraVexBoardModel.swift b/TetraVexKit/TetraVexBoardModel.swift
index c23cfad..8be777c 100644
--- a/TetraVexKit/TetraVexBoardModel.swift
+++ b/TetraVexKit/TetraVexBoardModel.swift
@@ -25,6 +25,10 @@ open class TetraVexBoardModel {
             return false
         }
         
+        if board[x][y] != nil {
+            return false
+        }
+        
         var leftP : PieceModel? = nil
         if 0 ... board.count-1 ~= x-1 && 0 ... board[0].count-1 ~= y {
             leftP = board[x-1][y]
diff --git a/TetraVexKitTests/TetraVexKitTests.swift b/TetraVexKitTests/TetraVexKitTests.swift
index e40f6ca..866438b 100644
--- a/TetraVexKitTests/TetraVexKitTests.swift
+++ b/TetraVexKitTests/TetraVexKitTests.swift
@@ -59,6 +59,8 @@ class TetraVexKitTests: XCTestCase {
         let board = TetraVexBoardModel(width: 2, height: 2)
         var r = board.addPieceToBoard(a, x: 0, y: 1)
         XCTAssert(r == true)
+        r = board.addPieceToBoard(b, x: 0, y: 1)
+        XCTAssert(r == false)
         r = board.addPieceToBoard(b, x: 1, y: 1)
         XCTAssert(r == true)
         r = board.addPieceToBoard(c, x: 1, y: 0)