The output of the two following statements illustrates the problem:

$.param({ 'test': 5 }); // "test=5"
$.param({ 'test': new Number(5) }); // ""

The problem is the that the function buildParams uses the test typeof obj === 'object' to test whether a value is a deep object or not. However, if the value is a wrapped number or boolean, then they will return 'object' as well, as this will result in the field not being included in the resulting parameter string.