hook_update_N must return an array

Updating this module produced these errors:

warning: array_merge() [function.array-merge]: Argument #2 is not an array in update.php on line 174.
warning: Invalid argument supplied for foreach() in update.php on line 338.

I suspect they are caused by this code not returning anything:

/**
* Delete variable 'module_filter_autocomplete'.
*/
function module_filter_update_6100() {
variable_del('module_filter_autocomplete');
}