updateParameter() python toolbox

I can’t update parameters in my Python Toolbox and I don’t understand why.
Someone can help me with this example code?

class Tool(object):
    def __init__(self):
        self.label = "Tool"
        self.description = ""
        self.canRunInBackground = False

    def getParameterInfo(self):
        p0=arcpy.Parameter(
            displayName="par0",
            name="n0",
            datatype="GPString",
            parameterType="Required",
            direction="Input")

        p1=arcpy.Parameter(
            displayName="par1",
            name="n1",
            datatype="GPString",
            parameterType="Optional",
            direction="Input")

        params=[p0,p1]
        return params

    def isLicensed(self):
        return True

    def updateParameter(self, parameters):
        if parameters[0].value:
            if parameters[0].value == "test":
                parameters[1].enabled = False
            else:
                parameters[1].enabled = True
        return

    def updateMessages(self, parameters):
        return

    def execute(self, parameters, messages):
        return