Longrightarrow does not work properly in mathastext.sty

My codes are follows:

documentclass{book}
usepackage[T1]{fontenc}%
usepackage{times}
usepackage[defaultmathsizes,italic]{mathastext}%
usepackage{mathtools}
begin{document}


begin{align*}
x in L &Longrightarrow big|vec{a}_x cdot mathcal{U} vec{a}_xbig|^2 geq 1 - epsilon(n),
x notin L &Longrightarrow big|vec{a}_x cdot mathcal{U} vec{a}_xbig|^2 leq epsilon(n).
end{align*}

end{document}

This works without error, but the output of Longrightarrow looks like kerning of = sign and leftarrow, and it looks very odd:

I’m using MikTeX 2.9 PDFLaTeX on Windows7 system, and I’m in need to use this same setup with the package mathastext, please suggest…