FFI TypeError

[pyrareae]

On running the examples (running on Linux 64bit) I get the following traceback/error:

Traceback (most recent call last):
        15: from histogram.rb:4:in `<main>'
        14: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:39:in `require'
        13: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:135:in `rescue in require'
        12: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:135:in `require'
        11: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/libui.rb:3:in `<top (required)>'
        10: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/libui.rb:3:in `require_relative'
         9: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/ext.rb:3:in `<top (required)>'
         8: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/ext.rb:4:in `<module:LibUI>'
         7: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/ext.rb:120:in `<module:Ext>'
         6: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/libui-ruby-1.0.1/lib/ext.rb:121:in `<class:AreaMouseEvent>'
         5: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/struct.rb:218:in `layout'
         4: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/struct.rb:306:in `array_layout'
         3: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/struct.rb:266:in `find_field_type'
         2: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/struct.rb:272:in `find_type'
         1: from /home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/library.rb:585:in `find_type'
/home/lunar/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/ffi-1.11.1/lib/ffi/types.rb:69:in `find_type': unable to resolve type 'uintmax_t' (TypeError)