Hi.
I am writing a functional test.
For html
<button type="submit" class="btn btn-success" name="start_game" value="yes">Start new game</button>
my test code
$I->seeInField(['name' => 'start_game'], 'yes');
produce this error.
Digging into your code I found that I was not supposed to try it with button input.
And this line of your library has mistake. $nodes is an object which has no __toString(), it cannot be inserted into string