In a debug build, I get the assert shown in the attached image.  After cancelling the assert dialog and performing a second paste operation, the crash yields the following stack trace in gdb:

(gdb) bt
#0  0x00000000070479c3 in SCH_BASE_FRAME::HandleBlockBegin (this=0x3fb61e50,
    aDC=0x0, aKey=6, aPosition=..., aExplicitCommand=0)
    at C:/msys64/home/wstambaugh/src/kicad-trunk/eeschema/sch_base_frame.cpp:542
#1  0x000000000709f593 in SCH_EDIT_FRAME::Process_Special_Functions (this=
    0x3fb61e50, event=...)
    at C:/msys64/home/wstambaugh/src/kicad-trunk/eeschema/schedit.cpp:148
#2  0x000000006a4c2f24 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#3  0x000000006a62cd42 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#4  0x000000006a62ce48 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#5  0x000000006a62d254 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#6  0x00000000071832b9 in EDA_BASE_FRAME::ProcessEvent (this=0x3fb61e50,
    aEvent=...)
    at C:/msys64/home/wstambaugh/src/kicad-trunk/common/eda_base_frame.cpp:173
#7  0x000000006a62d003 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#8  0x000000006a62d321 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#9  0x0000000004d9449f in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#10 0x000000006a62d05b in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#11 0x0000000004d41f8d in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#12 0x0000000004bdea70 in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#13 0x00007ff843586d41 in USER32!CallWindowProcW ()
   from C:\WINDOWS\System32\user32.dll
#14 0x00007ff843586a1c in USER32!CallWindowProcW ()
   from C:\WINDOWS\System32\user32.dll
#15 0x00007ff8435904d3 in USER32!IsWindowVisible ()
   from C:\WINDOWS\System32\user32.dll
#16 0x00007ff84460e6b4 in ntdll!KiUserCallbackDispatcher ()
   from C:\WINDOWS\SYSTEM32\ntdll.dll
#17 0x00007ff8416712a4 in win32u!NtUserTranslateAccelerator ()
   from C:\WINDOWS\System32\win32u.dll
#18 0x0000000004bafdcb in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#19 0x00000000072d31c4 in wxFrame::MSWTranslateMessage (this=0x3fb61e50,
    msg=0x430f890) at C:/msys64/mingw64/include/wx-3.0/wx/msw/frame.h:94
#20 0x0000000004bd6142 in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#21 0x0000000004bd6071 in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#22 0x0000000004bd5e8b in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#23 0x000000006a4fc6d0 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#24 0x000000006a4fc339 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#25 0x000000006a4c68fa in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#26 0x00000000004394d2 in APP_SINGLE_TOP::OnRun (this=0x6902d00)
    at C:/msys64/home/wstambaugh/src/kicad-trunk/common/single_top.cpp:186
#27 0x000000006a547977 in ?? ()
   from C:\msys64\mingw64\bin\wxbase30u_gcc_custom.dll
#28 0x0000000004aaa24b in ?? ()
   from C:\msys64\mingw64\bin\wxmsw30u_core_gcc_custom.dll
#29 0x00000000004016ec in WinMain (hInstance=0x400000, hPrevInstance=0x0,
    nCmdShow=10)
    at C:/msys64/home/wstambaugh/src/kicad-trunk/common/single_top.cpp:260
#30 0x00000000004618b2 in main (flags=<optimized out>,
    cmdline=<optimized out>, inst=<optimized out>)
    at C:/repo/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/crt0_c.c:18
#31 0x00000000004013a5 in __tmainCRTStartup ()
    at C:/repo/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:339
#32 0x00000000004014db in WinMainCRTStartup ()
    at C:/repo/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/crtexe.c:195