Cannot use font Seravek's small cap feature

[notcome]

I am using XeLaTeX & fontspec in TexLive on OS X Yosemite.

Seravek comes with OS X Mavericks.

The problem is that I could not use font Seravek's small caps feature. However, if I define a font family "Seravek ExtraLight" directly, small caps will work.

Here is a simplified code:

\documentclass{article}
\usepackage{fontspec}
\newfontfamily \seravek[BoldFont = * ExtraLight]{Seravek}
\newfontfamily \seravekEL{Seravek ExtraLight}
\newfontfamily \avenir{Avenir Next}

\begin{document}
\seravek \scshape Small Caps Testing

\seravek \scshape \textbf{Small Caps Testing}

\seravekEL \scshape Small Caps Testing

\avenir \scshape Small Caps Testing

\end{document}

The output:

The log info about fontspec indicated that fontspec failed to load small caps for Seravek:

. fontspec info: "defining-font"
. 
. Font family 'Seravek(0)' created for font 'Seravek' with options [].
. 
. This font family consists of the following shapes:
. 
. * 'normal' with NFSS spec.:
. <->"Seravek/ICU:"
. 
. * 'bold' with NFSS spec.:
. <->"Seravek ExtraLight/ICU:"
. 
. * 'italic' with NFSS spec.:
. <->"Seravek/I/ICU:"
. 
. * 'bold italic' with NFSS spec.:
. <->"Seravek ExtraLight/I/ICU:"
.................................................
\g_fontspec_family_Seravek ExtraLight_int=\count124
.................................................
. fontspec info: "defining-font"
. 
. Font family 'SeravekExtraLight(0)' created for font 'Seravek ExtraLight'
. with options [].
. 
. This font family consists of the following shapes:
. 
. * 'normal' with NFSS spec.:
. <->"Seravek ExtraLight/ICU:script=latn;language=DFLT;"
. 
. * 'small caps' with NFSS spec.:
. <->"Seravek ExtraLight/ICU:script=latn;language=DFLT;+smcp;"
. 
. * 'bold' with NFSS spec.:
. <->"Seravek ExtraLight/B/ICU:script=latn;language=DFLT;"
. 
. * 'italic' with NFSS spec.:
. <->"Seravek ExtraLight/I/ICU:script=latn;language=DFLT;"
. 
. * 'italic small caps' with NFSS spec.:
. <->"Seravek ExtraLight/I/ICU:script=latn;language=DFLT;+smcp;"
. 
. * 'bold italic' with NFSS spec.:
. <->"Seravek ExtraLight/BI/ICU:script=latn;language=DFLT;"
.................................................
\g_fontspec_family_Avenir Next_int=\count125
.................................................
. fontspec info: "defining-font"
. 
. Font family 'AvenirNext(0)' created for font 'Avenir Next' with options [].
. 
. This font family consists of the following shapes:
. 
. * 'normal' with NFSS spec.:
. <->"Avenir Next/ICU:script=latn;language=DFLT;"
. 
. * 'small caps' with NFSS spec.:
. <->"Avenir Next/ICU:script=latn;language=DFLT;+smcp;"
. 
. * 'bold' with NFSS spec.:
. <->"Avenir Next/B/ICU:script=latn;language=DFLT;"
. 
. * 'bold small caps' with NFSS spec.:
. <->"Avenir Next/B/ICU:script=latn;language=DFLT;+smcp;"
. 
. * 'italic' with NFSS spec.:
. <->"Avenir Next/I/ICU:script=latn;language=DFLT;"
. 
. * 'italic small caps' with NFSS spec.:
. <->"Avenir Next/I/ICU:script=latn;language=DFLT;+smcp;"
. 
. * 'bold italic' with NFSS spec.:
. <->"Avenir Next/BI/ICU:script=latn;language=DFLT;"
. 
. * 'bold italic small caps' with NFSS spec.:
. <->"Avenir Next/BI/ICU:script=latn;language=DFLT;+smcp;"
.................................................

Both XeLaTeX and LuaLaTeX has been tested. Every module is up-to-date.

Where might the problem be? Is it a bug of fontspec?