diff --git a/Connect4/connect4_lib/Sources/connect4_lib/Board.swift b/Connect4/connect4_lib/Sources/connect4_lib/Board.swift
index 76a1023..a81870a 100644
--- a/Connect4/connect4_lib/Sources/connect4_lib/Board.swift
+++ b/Connect4/connect4_lib/Sources/connect4_lib/Board.swift
@@ -28,6 +28,9 @@ public struct Board : CustomStringConvertible {
             for tile in row {
                 if tile != nil {
                     nbFree -= 1
+                    if (tile != 1 && tile != 2) {
+                        return nil
+                    }
                 }
             }
         }
diff --git a/Connect4/connect4_lib/Tests/connect4_libTests/BoardTest.swift b/Connect4/connect4_lib/Tests/connect4_libTests/BoardTest.swift
index 9ba40d6..0420a5c 100644
--- a/Connect4/connect4_lib/Tests/connect4_libTests/BoardTest.swift
+++ b/Connect4/connect4_lib/Tests/connect4_libTests/BoardTest.swift
@@ -39,11 +39,12 @@ final class BoardTest: XCTestCase {
             XCTAssertEqual(orig.count, board?.nbRows)
         }
         
-        expect(withGrid: [[0, 1, 2], [0, 0, 0], [0, 0, 0]], shouldNotBeNil: true)
+        expect(withGrid: [[nil, nil, nil], [nil, nil, nil], [nil, 1, 2]], shouldNotBeNil: true)
         expect(withGrid: [], shouldNotBeNil: false)
         expect(withGrid: [[], []], shouldNotBeNil: false)
-        expect(withGrid: [[0, 1], [0, 0]], shouldNotBeNil: false)
-        expect(withGrid: [[0, 1, 2], [0, 0, 0, 0]], shouldNotBeNil: false)
+        expect(withGrid: [[nil, nil], [nil, 1]], shouldNotBeNil: false)
+        expect(withGrid: [[nil, nil, nil], [nil, nil, 1, 2]], shouldNotBeNil: false)
+        expect(withGrid: [[nil, nil, nil], [nil, nil, nil], [nil, 1, 3]], shouldNotBeNil: false)
     }
     
     func testIsFull() throws {