diff --git a/src/model.c b/src/model.c
index c8112f8..939031b 100644
--- a/src/model.c
+++ b/src/model.c
@@ -134,18 +134,27 @@ e_location get_next_outer_position(s_utictactoe *p_uttt) {
 }
 
 e_status is_move_valid(s_utictactoe *p_uttt, s_move *p_move) {
+    
+    // the party is not over?
     if ( p_uttt->outer_tictactoe->winner != NOBODY ) {
         return NO;
     }
     
+    // this is the awaited player that played?
     if ( get_next_player_to_play(p_uttt) != p_move->player ) {
         return NO;
     }
     
+    // the outer position is correct?
     if ( get_next_outer_position(p_uttt) != FREE 
             && get_next_outer_position(p_uttt) != p_move->outer_position) {
         return NO;
     }
+    
+    // position in the inner position is free?
+    if ( p_uttt->inner_tictactoes[p_move->outer_position]->content[p_move->inner_position] != NOBODY ) {
+        return NO;
+    }
 
     return YES;
 }