How to use guile-cairo

I try to to use this extension but got this error:

guile> (load-from-path "cairo")

guile> (load-from-path "cairo")
guile> (cairo-pdf-surface-create "foo.pdf" 100.0 100.0) 

Backtrace:
In standard input:
   6: 0* [cairo-pdf-surface-create "foo.pdf" 100.0 {100.0}]

standard input:6:1: In procedure cairo-pdf-surface-create in expression
(cairo-pdf-surface-create "foo.pdf" 100.0 ...):
standard input:6:1: Wrong type (expecting string): 100.0
ABORT: (wrong-type-arg)

guile> (cairo-pdf-surface-create "foo.pdf" "100.0" "100.0")

Backtrace:
In standard input:
   7: 0* [cairo-pdf-surface-create "foo.pdf" {"100.0"} "100.0"]

standard input:7:1: In procedure cairo-pdf-surface-create in expression
(cairo-pdf-surface-create "foo.pdf" "100.0" ...):
standard input:7:1: Wrong type (expecting real number): "100.0"
ABORT: (wrong-type-arg)

I'm using Ubuntu 10.4 / guile 1.8.7

Thanks in advance.