diff --git a/src/main/java/dotsandboxes/DotsAndBoxesGrid.java b/src/main/java/dotsandboxes/DotsAndBoxesGrid.java
index a9e7c5b6639e8a2a8728809d9b17c8d155baf9b6..43210def4ee15e3d626bdc24f0b830907992d9c7 100644
--- a/src/main/java/dotsandboxes/DotsAndBoxesGrid.java
+++ b/src/main/java/dotsandboxes/DotsAndBoxesGrid.java
@@ -110,9 +110,13 @@ public class DotsAndBoxesGrid {
             return false;
         }
 
-        // A box is complete if the north and south horizontals and the east and west verticals have all been drawn.
-        // FIXME: You'll need to fix this code (after writing a test first).
-        return true;
+        // Could be shorter but this is more readable
+        boolean north = horizontals[x][y];
+        boolean south = horizontals[x][y + 1];
+        boolean west = verticals[x][y];
+        boolean east = verticals[x + 1][y];
+
+        return north && south && west && east;
     }
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
@@ -139,7 +143,8 @@ public class DotsAndBoxesGrid {
             throw new IndexOutOfBoundsException(String.format("y was %d, which is out of range. Range is 0 to %d", y, height));
         }
 
-        // FIXME: You need to throw an exception if the line was already drawn.
+        // You need to throw an exception if the line was already drawn.
+        if (horizontals[x][y]) throw new IllegalArgumentException("Line is already taken.");
 
         this.horizontals[x][y] = true;
 
@@ -171,6 +176,7 @@ public class DotsAndBoxesGrid {
         }
 
         // You need to throw an exception if the line was already drawn.
+        if (verticals[x][y]) throw new IllegalArgumentException("Line is already taken.");
 
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes
diff --git a/src/test/java/dotsandboxes/DotsAndBoxesGridTest.java b/src/test/java/dotsandboxes/DotsAndBoxesGridTest.java
index ee51c59dce963c5ba778aeff7be9657df8a55d91..d7454fb669ce70e3e6915d0b657406cfdc261cd4 100644
--- a/src/test/java/dotsandboxes/DotsAndBoxesGridTest.java
+++ b/src/test/java/dotsandboxes/DotsAndBoxesGridTest.java
@@ -78,7 +78,7 @@ public class DotsAndBoxesGridTest {
         DotsAndBoxesGrid grid = new DotsAndBoxesGrid(3, 3, 2);
         grid.drawHorizontal(1, 1, 1);
 
-        Exception exception = assertThrows(IllegalStateException.class, () -> {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> {
             grid.drawHorizontal(1, 1, 2);
         });