Source

File: wp-includes/Requests/Utility/CaseInsensitiveDictionary.php

public function offsetSet($key, $value) {
		if ($key === null) {
			throw new Requests_Exception('Object is a dictionary, not a list', 'invalidset');
		}

		$key = strtolower($key);
		$this->data[$key] = $value;
	}