[pdftex] Vietnamese fails to print some particular phrase

[Prabhu]

In my .tex file, I use T5 fontenc to produce vietnamese font. I can able to
generate output (using pdflatex) with most of the phrase text. But some
phrases throws a error.  can some one help in pointing the cause of the
error.. Thanks.



The probalamatic phrase is

"Tham khảo Phần 3 trong Phiếu an toàn hóa chất này về
phân loại theo GHS"

And the error is

! Package ucs Error: Please activate option 'combine'.
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