XELATEX error : l.52 make_char_active:n

Question

I am using WinEdt 7.0 in WINDOWS 7. I was trying run a tex file using XeLaTeX. The error reads as follows:

! Undefined control sequence
l.52 \char_make_active:n{"20}%

Please suggest on this.

UPDATE by texnic: Since I am having exactly the same issue now, I thought I might join this discussion. The MWE is, as suggested by Joseph, simply

\listfiles
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\end{document}

The command is xelatex -synctex=1 -interaction=nonstopmode %.tex.

And the log:

This is XeTeX, Version 3.1415926-2.3-0.9997.5 (MiKTeX 2.9) (preloaded format=xelatex 2012.9.17)  17 SEP 2012 10:08
entering extended mode
**fontspec_bug.tex
("C:\Users\Kotya\Documents\Dropbox\job search\fontspec_bug.tex"
LaTeX2e 
Babel  and hyphenation patterns for english, afrikaans, ancientgreek, ar
abic, armenian, assamese, basque, bengali, bokmal, bulgarian, catalan, coptic, 
croatian, czech, danish, dutch, esperanto, estonian, farsi, finnish, french, ga
lician, german, german-x-2009-06-19, greek, gujarati, hindi, hungarian, iceland
ic, indonesian, interlingua, irish, italian, kannada, kurmanji, lao, latin, lat
vian, lithuanian, malayalam, marathi, mongolian, mongolianlmc, monogreek, ngerm
an, ngerman-x-2009-06-19, nynorsk, oriya, panjabi, pinyin, polish, portuguese, 
romanian, russian, sanskrit, serbian, slovak, slovenian, spanish, swedish, swis
sgerman, tamil, telugu, turkish, turkmen, ukenglish, ukrainian, uppersorbian, u
senglishmax, welsh, loaded.
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\base\article.cls"
Document Class: article 2007/10/19 v1.4h Standard LaTeX document class
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\base\size10.clo"
File: size10.clo 2007/10/19 v1.4h Standard LaTeX file (size option)
)
\c@part=\count80
\c@section=\count81
\c@subsection=\count82
\c@subsubsection=\count83
\c@paragraph=\count84
\c@subparagraph=\count85
\c@figure=\count86
\c@table=\count87
\abovecaptionskip=\skip41
\belowcaptionskip=\skip42
\bibindent=\dimen102
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\fontspec\fontspec.sty"
Package: fontspec 2011/02/26 v2.1f Advanced font selection for XeLaTeX/LuaLaTeX


("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\expl3.sty"
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3names.sty"
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3bootstrap.sty"
Package: l3bootstrap 2012/07/16 v3991 L3 Experimental bootstrap code
)
Package: l3names 2012/07/16 v3990 L3 Namespace for primitives
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\misc\etex.sty"
Package: etex 1998/03/26 v2.0 eTeX basic definition package (PEB)
\et@xins=\count88
)
Package: expl3 2012/09/05 v4205 L3 Experimental code bundle wrapper

("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3basics.sty"
Package: l3basics 2012/08/27 v4144 L3 Basic definitions
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3expan.sty"
Package: l3expan 2012/08/28 v4149 L3 Argument expansion
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3tl.sty"
Package: l3tl 2012/09/05 v4201 L3 Token lists
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3seq.sty"
Package: l3seq 2012/08/14 v4092 L3 Sequences and stacks
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3int.sty"
Package: l3int 2012/08/17 v4121 L3 Integers
\c_max_int=\count89
\l_tmpa_int=\count90
\l_tmpb_int=\count91
\g_tmpa_int=\count92
\g_tmpb_int=\count93
\l_tmpc_int=\count94
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3quark.sty"
Package: l3quark 2012/08/08 v4065 L3 Quarks
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3prg.sty"
Package: l3prg 2012/08/16 v4114 L3 Control structures
\g__prg_map_int=\count95
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3clist.sty"
Package: l3clist 2012/08/15 v4102 L3 Comma separated lists
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3token.sty"
Package: l3token 2012/08/08 v4069 L3 Experimental token manipulation
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3prop.sty"
Package: l3prop 2012/09/03 v4190 L3 Property lists
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3msg.sty"
Package: l3msg 2012/08/28 v4147 L3 Messages
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3file.sty"
Package: l3file 2012/08/15 v4101 L3 File and I/O operations
\l__ior_stream_int=\count96
\l_iow_line_count_int=\count97
\l__iow_target_count_int=\count98
\l__iow_current_line_int=\count99
\l__iow_current_word_int=\count100
\l__iow_current_indentation_int=\count101
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3skip.sty"
Package: l3skip 2012/08/17 v4121 L3 Dimensions and skips
\l_tmpa_dim=\dimen103
\l_tmpb_dim=\dimen104
\g_tmpa_dim=\dimen105
\g_tmpb_dim=\dimen106
\l_tmpa_skip=\skip43
\l_tmpb_skip=\skip44
\g_tmpa_skip=\skip45
\g_tmpb_skip=\skip46
\c_zero_muskip=\muskip10
\c_max_muskip=\muskip11
\l_tmpa_muskip=\muskip12
\l_tmpb_muskip=\muskip13
\g_tmpa_muskip=\muskip14
\g_tmpb_muskip=\muskip15
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3keys.sty"
Package: l3keys 2012/07/16 v3991 L3 Experimental key-value interfaces
\g__keyval_level_int=\count102
\l_keys_choice_int=\count103
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3fp.sty"
Package: l3fp 2012/08/28 v4151 L3 Floating points
\c__fp_leading_shift_int=\count104
\c__fp_middle_shift_int=\count105
\c__fp_trailing_shift_int=\count106
\c__fp_big_leading_shift_int=\count107
\c__fp_big_middle_shift_int=\count108
\c__fp_big_trailing_shift_int=\count109
\c__fp_Bigg_leading_shift_int=\count110
\c__fp_Bigg_middle_shift_int=\count111
\c__fp_Bigg_trailing_shift_int=\count112
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3box.sty"
Package: l3box 2012/07/16 v3991 L3 Experimental boxes
\l_tmpb_box=\box26
\g_tmpa_box=\box27
\g_tmpb_box=\box28
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3coffins.sty"
Package: l3coffins 2012/08/14 v4092 L3 Coffin code layer
\l__coffin_internal_box=\box29
\l__coffin_internal_dim=\dimen107
\l__coffin_offset_x_dim=\dimen108
\l__coffin_offset_y_dim=\dimen109
\l__coffin_x_dim=\dimen110
\l__coffin_y_dim=\dimen111
\l__coffin_x_prime_dim=\dimen112
\l__coffin_y_prime_dim=\dimen113
\c_empty_coffin=\box30
\l__coffin_aligned_coffin=\box31
\l__coffin_aligned_internal_coffin=\box32
\l_tmpa_coffin=\box33
\l_tmpb_coffin=\box34
\l__coffin_display_coffin=\box35
\l__coffin_display_coord_coffin=\box36
\l__coffin_display_pole_coffin=\box37
\l__coffin_display_offset_dim=\dimen114
\l__coffin_display_x_dim=\dimen115
\l__coffin_display_y_dim=\dimen116
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3color.sty"
Package: l3color 2012/08/29 v4156 L3 Experimental color support
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3luatex.sty"
Package: l3luatex 2012/08/03 v4049 L3 Experimental LuaTeX-specific functions
\g__cctab_allocate_int=\count113
\g__cctab_stack_int=\count114
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3kernel\l3candidates.sty"
Package: l3candidates 2012/05/12 v3633 L3 Experimental additions to l3kernel
\l__box_top_dim=\dimen117
\l__box_bottom_dim=\dimen118
\l__box_left_dim=\dimen119
\l__box_right_dim=\dimen120
\l__box_top_new_dim=\dimen121
\l__box_bottom_new_dim=\dimen122
\l__box_left_new_dim=\dimen123
\l__box_right_new_dim=\dimen124
\l__box_internal_box=\box38
\l__coffin_bounding_shift_dim=\dimen125
\l__coffin_left_corner_dim=\dimen126
\l__coffin_right_corner_dim=\dimen127
\l__coffin_bottom_corner_dim=\dimen128
\l__coffin_top_corner_dim=\dimen129
\l__coffin_scaled_total_height_dim=\dimen130
\l__coffin_scaled_width_dim=\dimen131
))
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\l3packages\xparse\xparse.sty"
Package: xparse 2012/09/05 v4205 L3 Experimental document command parser
\l__xparse_current_arg_int=\count115
\l__xparse_m_args_int=\count116
\l__xparse_mandatory_args_int=\count117
\l__xparse_processor_int=\count118
\l__xparse_v_nesting_int=\count119
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\generic\binhex\binhex.tex")
\l_fontspec_script_int=\count120
\l_fontspec_language_int=\count121
\l_fontspec_strnum_int=\count122
Variant \tl_gset:cV already defined; not changing it on line 83

("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\tools\calc.sty"
Package: calc 2007/08/22 v4.3 Infix arithmetic (KKT,FJ)
\calc@Acount=\count123
\calc@Bcount=\count124
\calc@Adimen=\dimen132
\calc@Bdimen=\dimen133
\calc@Askip=\skip47
\calc@Bskip=\skip48
LaTeX Info: Redefining \setlength on input line 76.
LaTeX Info: Redefining \addtolength on input line 77.
\calc@Ccount=\count125
\calc@Cskip=\skip49
)
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\xkeyval\xkeyval.sty"
Package: xkeyval 2008/08/13 v2.6a package option processing (HA)

("C:\Program Files (x86)\MiKTeX 2.9\tex\generic\xkeyval\xkeyval.tex"
\XKV@toks=\toks14
\XKV@tempa@toks=\toks15
\XKV@depth=\count126
File: xkeyval.tex 2008/08/13 v2.6a key=value parser (HA)

("C:\Program Files (x86)\MiKTeX 2.9\tex\generic\xkeyval\keyval.tex")))
("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\fontspec\fontspec-patches.sty"
Package: fontspec-patches 2011/02/26 v2.1f Advanced font selection for XeLaTeX/
LuaLaTeX

("C:\Program Files (x86)\MiKTeX 2.9\tex\latex\base\fixltx2e.sty"
Package: fixltx2e 2006/09/13 v1.1m fixes to LaTeX
LaTeX Info: Redefining \em on input line 420.
)
LaTeX Info: Redefining \em on input line 22.
LaTeX Info: Redefining \emph on input line 29.
LaTeX Info: Redefining \- on input line 32.

! Undefined control sequence.
l.52 \char_make_active:n
                        {"20}%
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.


! LaTeX Error: Missing \begin{document}.

(THE REST OF THE LOG SKIPPED BY texnic)

Answer 1

Based on the comments to the question, the solution is probably to update the fontspec package.

1. Do Start / All programs / MikTeX / Maintenance (I used admin, however make sure to check this discussion) / Update
2. Choose "I want to ... remote package repository" and specify the nearest or any other repository.
3. Wait till MikTeX finds all packages it can update (takes a while).
4. During the first run, MikTeX decided to update only itself: all other outdated packages, including fontspec, were not selectable.
5. After updating MikTeX, re-run the updater. This time MikTeX automatically selects all outdated packages. I decided to update all of them. (Again, this takes a while.)