how to set “aspect-ratio” GstFraction * in element aspectratiocrop

[cfd new]

I have a pipeline with element aspectratiocrop. When I tried to set aspect-ratio withstd::string crop_str( "4/3" );
auto crop = gst_element_factory_make( "aspectratiocrop", nullptr );
g_object_set( G_OBJECT( crop ), "aspect-ratio", crop_str.c_str(), nullptr );
the app complains about 
passed '0' as denometer for GstFraction. 
How to set it properly?
   Joe
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