QApplication gives assertion while parsing '/usr/share/X11/locale/iso8859-1/Compose' in debug release with LANG=C

$ LANG=en_US ./qt5lang
Locale: "C" 
^C

$ LANG=C ./qt5lang
Locale: "C" 
ASSERT: "uint(i) < uint(size())" in file ../../../../include/QtCore/../../src/corelib/tools/qstring.h, line 739

#0  0x000000343de32925 in raise () from /lib64/libc.so.6
#1  0x000000343de34105 in abort () from /lib64/libc.so.6
#2  0x00007ffff77efb44 in qt_message_fatal (context=..., message=...) at global/qlogging.cpp:979
#3  0x00007ffff77ed7b8 in QMessageLogger::fatal (this=0x7fffffffd4d0, 
    msg=0x7ffff7b63130 "ASSERT: \"%s\" in file %s, line %d") at global/qlogging.cpp:384
#4  0x00007ffff77e8cc0 in qt_assert (assertion=0x7ffff52ae585 "uint(i) < uint(size())", 
    file=0x7ffff52ae548 "../../../../include/QtCore/../../src/corelib/tools/qstring.h", line=739)
    at global/qglobal.cpp:2096
#5  0x00007ffff5286047 in QString::at (this=0x7fffffffd5a0, i=0)
    at ../../../../include/QtCore/../../src/corelib/tools/qstring.h:739
#6  0x00007ffff5285335 in TableGenerator::keysymToUtf8 (this=0x7fffffffddb0, sym=161)
    at generator/qtablegenerator.cpp:353
#7  0x00007ffff52855d9 in TableGenerator::parseKeySequence (this=0x7fffffffddb0, 
    line=0x7fffffffd700 "<Multi_key> <exclam> <exclam>\t\t: \"\\241\"\texclamdown\n")
    at generator/qtablegenerator.cpp:416
#8  0x00007ffff5284f8a in TableGenerator::parseComposeFile (this=0x7fffffffddb0, composeFile=0x7fffffffdb30)
    at generator/qtablegenerator.cpp:297
#9  0x00007ffff5284d93 in TableGenerator::processFile (this=0x7fffffffddb0, composeFileName=...)
    at generator/qtablegenerator.cpp:270
#10 0x00007ffff528436e in TableGenerator::findComposeFile (this=0x7fffffffddb0) at generator/qtablegenerator.cpp:125
#11 0x00007ffff5283e17 in TableGenerator::TableGenerator (this=0x7fffffffddb0) at generator/qtablegenerator.cpp:66
#12 0x00007ffff528234a in QComposeInputContext::QComposeInputContext (this=0x683590)
    at qcomposeplatforminputcontext.cpp:92
#13 0x00007ffff5281b6e in QComposePlatformInputContextPlugin::create (this=0x66d170, system=..., paramList=...)
    at main.cpp:64
#14 0x00007ffff6603d8d in qLoadPlugin1<QPlatformInputContext, QPlatformInputContextPlugin, QStringList> (loader=
    0x7ffff6d238f0, key=..., parameter1=...)
    at ../../include/QtCore/5.2.1/QtCore/private/../../../../../src/corelib/plugin/qfactoryloader_p.h:115
#15 0x00007ffff6603703 in QPlatformInputContextFactory::create (key=...) at kernel/qplatforminputcontextfactory.cpp:73
#16 0x00007ffff66038a8 in QPlatformInputContextFactory::create () at kernel/qplatforminputcontextfactory.cpp:99
#17 0x00007ffff5f488fd in QXcbIntegration::initialize (this=0x64d2e0) at qxcbintegration.cpp:303
#18 0x00007ffff66130d4 in QGuiApplicationPrivate::eventDispatcherReady (this=0x64cdf0)
    at kernel/qguiapplication.cpp:1056
#19 0x00007ffff7a7976e in QCoreApplication::init (this=0x7fffffffe0c0) at kernel/qcoreapplication.cpp:720
#20 0x00007ffff7a7944f in QCoreApplication::QCoreApplication (this=0x7fffffffe0c0, p=...)
    at kernel/qcoreapplication.cpp:638
#21 0x00007ffff6610fef in QGuiApplication::QGuiApplication (this=0x7fffffffe0c0, p=...)
    at kernel/qguiapplication.cpp:492
#22 0x00007ffff6ebfce2 in QApplication::QApplication (this=0x7fffffffe0c0, argc=@0x7fffffffe0bc, argv=0x7fffffffe218, 
    _internal=328193) at kernel/qapplication.cpp:537
#23 0x0000000000400dea in main (argc=1, argv=0x7fffffffe218) at qt5lang.cpp:7