ncurses (v.5.1 of 20001009) in FreeBSD-current (version is specified above)
have ugly tgoto_internal() function
(file src/contrib/ncurses/ncurses/tinfo/lib_tgoto.c) which cannot deal
properly with string with length 0. screen (3.9.8 from ports r3 hours ago)
calls tgoto() with such empty string in CPutStr().

Repeated situation is running mutt (the popular text mode mail user agent)
with threaded grouping of letters in mailbox in screen's virtual terminal.
Following is diagnostics log.

Core file (to generate, set kern.sugid_coredump to 1):
-rw-------   1 netch  wheel   479232 Nov 24 00:59 screen.dbg.2704.core

(gdb) bt
#0  0x281521fc in kill () from /usr/lib/libc.so.4
#1  0x28190f26 in abort () from /usr/lib/libc.so.4
#2  0x804cced in CoreDump (sigsig=11) at screen.c:1413
#3  0xbfbfffac in ?? ()
#4  0x280c658d in tgoto () from /usr/lib/libncurses.so.5
#5  0x80809e8 in CPutStr (s=0x8094f4e "", c=48) at display.c:902
#6  0x80831cc in SetFont (new=48) at display.c:1789
#7  0x8083582 in SetRendition (mc=0x80ba324) at display.c:1855
#8  0x808c374 in LSetRendition (l=0x80b800c, r=0x80ba324) at layer.c:507
#9  0x8052f06 in DesignateCharset (c=48, n=0) at ansi.c:1517
#10 0x8051966 in DoESC (c=48, intermediate=40) at ansi.c:943
#11 0x8050788 in WriteString (wp=0x80b8000,
    buf=0xbfbfe295 "\e[?25l\e[1;1H\e[7m\e[37m\e[40m---Mutt: /var/mail/netch
599
K [Msgs:56 New:24 Post:5]", '-' <repeats 18 times>,
"(39%)---\e[2;1H\e[m\e[37m\e
[40m1   |\e[5C| Pavel Gulchouck\e[6C| Re: sendmail.cf.pl?\r\n2   |\e[5C|
Valenti
n Ne"..., len=1589) at ansi.c:546
#12 0x80689d5 in win_readev_fn (ev=0x80b8048, data=0x80b8000 "")
    at window.c:1768
#13 0x808e33e in sched () at sched.c:237
#14 0x804c78e in main (ac=0, av=0xbfbffb94) at screen.c:1255
#15 0x8049fb5 in _start ()

(gdb) f 5
#5  0x80809e8 in CPutStr (s=0x8094f4e "", c=48) at display.c:902
902           tputs(tgoto(s, 0, c), 1, DoAddChar);
(gdb) p s
$1 = 0x8094f4e ""
(gdb) p c
$2 = 48
(gdb) p DoAddChar
$3 = {int (int)} 0x8080918 <DoAddChar>
(gdb) f 4
#4  0x280c658d in tgoto () from /usr/lib/libncurses.so.5
(gdb) info f 4
Stack frame at 0xbfbfe0ec:
 eip = 0x280c658d in tgoto; saved eip 0x80809e8
 called by frame at 0xbfbfe118, caller of frame at 0xbfbfe0bc
 Arglist at 0xbfbfe0ec, args:
 Locals at 0xbfbfe0ec, Previous frame's sp is 0x0
 Saved registers:
  ebx at 0xbfbfe0d4, ebp at 0xbfbfe0ec, esi at 0xbfbfe0d8, edi at
0xbfbfe0dc,
  eip at 0xbfbfe0f0
(gdb) x/w 0xbfbfe0f4
0xbfbfe0f4:     0x08094f4e
(gdb) x/c 0x08094f4e
0x8094f4e <TermcapConst+1006>:  0 '\000'
(gdb) x/w 0xbfbfe0f8
0xbfbfe0f8:     0 '\000'
(gdb) x/w 0xbfbfe0fc
0xbfbfe0fc:     48 '0'

e.g., string=="", x==0, y==48

(gdb) disas 0x280c658d
Dump of assembler code for function tgoto:
0x280c6554 <tgoto>:     push   %ebp
0x280c6555 <tgoto+1>:   mov    %esp,%ebp
0x280c6557 <tgoto+3>:   sub    $0xc,%esp
0x280c655a <tgoto+6>:   push   %edi
0x280c655b <tgoto+7>:   push   %esi
0x280c655c <tgoto+8>:   push   %ebx
0x280c655d <tgoto+9>:   call   0x280c6562 <tgoto+14>
0x280c6562 <tgoto+14>:  pop    %ebx
0x280c6563 <tgoto+15>:  add    $0x2e112,%ebx
0x280c6569 <tgoto+21>:  mov    0x8(%ebp),%esi
(%esi <- string)
0x280c656c <tgoto+24>:  mov    0x10(%ebp),%edi
(%edi <- y)
0x280c656f <tgoto+27>:  add    $0xfffffff4,%esp
0x280c6572 <tgoto+30>:  push   %esi
(string)
0x280c6573 <tgoto+31>:  call   0x280c6044 <_nc_lib_traceatr+8>
(is_termcap(string).)
0x280c6578 <tgoto+36>:  add    $0x10,%esp
0x280c657b <tgoto+39>:  test   %al,%al
0x280c657d <tgoto+41>:  je     0x280c6590 <tgoto+60>
0x280c657f <tgoto+43>:  add    $0xfffffffc,%esp
0x280c6582 <tgoto+46>:  push   %edi
(y)
0x280c6583 <tgoto+47>:  mov    0xc(%ebp),%eax
0x280c6586 <tgoto+50>:  push   %eax
(x)
0x280c6587 <tgoto+51>:  push   %esi
(string)
0x280c6588 <tgoto+52>:  call   0x280c6088 <_nc_lib_traceatr+76>
(tgoto_internal(string,x,y))
---Type <return> to continue, or q <return> to quit---
0x280c658d <tgoto+57>:  jmp    0x280c659e <tgoto+74>

The bad function is tgoto_internal().
In case when string is empty, local variables "result" and "length"
keep garbage; code as "strcpy(result + used, BC);" and "result[used] = '\0';"
breaks program consistence.

Fix: This is quick-and-dirty workaround for screen (tested and considered working).
See parallel PR for possible fix for ncurses.
How-To-Repeat: 
See Description