pycairo error: “no current point defined” when trying to use TeeSurface

Question

I'm trying to use a cairo.TeeSurface (pycairo version '1.21.0') to redirect input to multiple surfaces, but I cannot make it work, here is my code:

>>> import cairo
>>> s1 = cairo.ImageSurface(cairo.FORMAT_ARGB32, 500,500)
>>> s2 = cairo.ImageSurface(cairo.FORMAT_ARGB32, 500,500)
>>> s3 = cairo.ImageSurface(cairo.FORMAT_ARGB32, 500,500)
>>> s = cairo.TeeSurface(s1)
>>> s.add(s2)
>>> s.add(s3)
>>> c = cairo.Context(s)

After above iniziatilation if I try to draw a line I get an error:

>>> c.rel_line_to(100,100)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
cairo.Error: no current point defined

I tried to set explicitly a point but I get the same error:

>>> c.move_to(10,10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
cairo.Error: no current point defined

For sure I must have misunderstood something, any clue would be appreciated.