Null error when no king in FEN

[RedDragonWebDesign]

When reading this FEN. Related to ChessRulebook

8/8/8/3pp3/4P3/3P1N2/PPP1BPPP/RNBQK2R w KQkq d6 0 5

`Fatal error: Uncaught TypeError: Argument 3 passed to ChessRulebook::square_is_attacked() must be an instance of ChessSquare, null given, called in D:\Dropbox\Code\chess\chess_v2.2 [before putting on Red Dragon Web Design]\models\ChessRulebook.php on line 675 and defined in D:\Dropbox\Code\chess\chess_v2.2 [before putting on Red Dragon Web Design]\models\ChessRulebook.php on line 794

TypeError: Argument 3 passed to ChessRulebook::square_is_attacked() must be an instance of ChessSquare, null given, called in D:\Dropbox\Code\chess\chess_v2.2 [before putting on Red Dragon Web Design]\models\ChessRulebook.php on line 675 in D:\Dropbox\Code\chess\chess_v2.2 [before putting on Red Dragon Web Design]\models\ChessRulebook.php on line 794`