The following vapi results in a valac assertion failure and invalid C code:

uuid.vapi:

namespace UUid {
    [SimpleType]
    [CCode (cname = "uuid_t", cheader_filename = "uuid.h")]
    struct Uuid {
	public void clear();
    }
}

Here's what I use to create the object:

main.vala:

using UUid;

static void main() {
    Uuid uuid = Uuid();

    uuid.clear();
}

Compiling results in the following:

darrenw@washington:~/src/uuid$ valac  --vapidir=. --pkg=uuid main.vala

** (valac:16872): CRITICAL **: vala_ccode_unary_expression_construct: assertion `expr != NULL' failed

** (valac:16872): CRITICAL **: vala_ccode_function_call_add_argument: assertion `expr != NULL' failed
/home/darrenw/src/uuid/main.vala.c: In function ‘_vala_main’:
/home/darrenw/src/uuid/main.vala.c:19: error: too few arguments to function ‘memset’
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)

As far as I can debug this, in CCodeBaseModule.visit_object_creation_expression(), not all code paths result in a value being assigned to 'instance'. When a new CCodeUnaryExpression is later constructed, the null instance value is passed and results in the exception.