If the selector you attempt to use scans for attributes that don't exist (e.g. $('div[@test$=end]') and there exists dividers without a test attribute) jquery throws a z has no properties error.

This appears to come from the following lines

if ((type == "" && !!z ||

type == "=" && z == m[5] ||

type == "!=" && z != m[5] ||

type == "^=" && z && !z.indexOf(m[5]) ||

type == "$=" && z.substr(z.length - m[5].length) == m[5] ||

(type == "*=" || type == "~=") && z.indexOf(m[5]) >= 0) ^ not )

tmp.push( a );

Where ^= checks z (&& z &&) whereas $= (and *= and ~=) assumes it is valid.