TimeFunction generally accepts a name as a string as its parameter. However, if timeFunction is the outer function in a nested function, or a series is passed in as an argument an error occurs, as it is expected a string type. An example would be:

time(removeBelowValue(metric, 0))

or

time(metric)

Although these usages don't meet the function definition:

timeFunction(name, step=60)

It appears that Graphite web does not error out, and instead just returns a series with no name.