Why does this execute method makes AN stop working As soon as I add the execute method, AN is gone when I start blender.

import bpy
from ... base_types.node import AnimationNode

class Foo(bpy.types.Node, AnimationNode):
    bl_idname = "an_Foo"
    bl_label = "Foo"


    def create(self):
        self.outputs.new("an_StringSocket", "Debug", "debug")


    def execute(self):
        debug = "foo"
        return debug


Its in a folder called serial where I have another node with the same problem.
It was inconsistent use of tab and spaces. I used tabs where the document was using 4 spaces before.

The console clearly report this.