Double uses augmentation - code generator NPE

yang model snippet:

grouping machine-types {
container types {
choice machine-type {
}
}
}

grouping machine-ext {
uses machine-types {
augment "types/machine-type" {
case type-a {
leaf id

{ type string; }
}
}
}
}

grouping machine-ext2 {
uses machine-ext {
augment "types/machine-type" {
case type-b {
leaf name { type string; }

}
}
}
}

unable to generate sources for grouping machine-ext2's augment "types/machine-type"