Smartapp handler runs when called but fails when scheduled

In my smartapp I defined a simple procedure/handler, If I call it directly updateCurrentWeather() or by runIn(60, updateCurrentWeather) it works fine, but if I schedule it to runEvery5Minutes(updateCurrentWeather) it fails.

error groovy.lang.MissingMethodException: No signature of method: script_app_Windows_Co_14fc18aa_4aa3_4861_8019_f21c6351a33f_ver_8_4.null() is applicable for argument types: () values: []
Possible solutions: url(), url(java.util.Map), dump(), now(), run(), dump()

The handler is calling an URL through xmlslurper, can I not do this ?