[error] LibCairo::Rectangle compiling demo

[nodrygo]

hello
first of all thank for this great work

trying the sample _gtksample.cr I got this error

crystal run samples/gtk_sample.cr 
Showing last frame. Use --error-trace for full trace.
In src/lib_cairo.cr:20:18
 20 | rectangles : LibCairo::Rectangle*
                   ^------------------
Error: undefined constant LibCairo::Rectangle

please note that crystal-gobject (another great work) samples work on my machine

is it a problem with my cairo release ?
my env. is

Linux Mint19  5.3.0-40-generic #32~18.04.1-Ubuntu SMP
libgtk 3.22.30 (standard installed)
libcairo, libcairo-gobject2   1.15.10

thanks for help

[nodrygo]

sorry bad list