"invalid ELF class: ELFCLASS32", but only when using GDB

I am trying to run dfhack in the debugger. I am using 64-bit Ubuntu Trusty, but Dwarf Fortress does not have a 64-bit build, so I am running 32-bit Dwarf Fortress. I installed gdb via apt-get install gdb:i386

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This is what happens when I run dfhack with gdb:

ben@australium:~/df_linux$ xvfb-run -a ./dfhack --gdb
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./libs/Dwarf_Fortress...(no debugging symbols found)...done.
(gdb) start
Temporary breakpoint 1 at 0x804bdec
Starting program: /home/ben/df_linux/libs/Dwarf_Fortress 
ERROR: ld.so: object './hack/libdfhack.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".

Temporary breakpoint 1, 0xf7598b53 in main ()
   from /home/ben/df_linux/libs/libgraphics.so
(gdb) 

      

        
        
        
      

    

This is what happens when I run it without gdb:

ben@australium:~/df_linux$ xvfb-run -a ./dfhack
Xlib:  extension "RANDR" missing on display ":99".
Loading bindings from data/init/interface.txt
New window size: 1280x720
Font size: 10x12
Resizing grid to 128x60
Resizing font to 10x12

Resetting textures
DFHack is ready. Have a nice day!
Type in '?' or 'help' for general help, 'ls' to see all commands.
[DFHack]# 

      

        
        
        
      

    

The executable, preloaded library and gdb are 32-bit:

ben@australium:~/df_linux$ readelf -h /home/ben/df_linux/libs/Dwarf_Fortress
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2 complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x804c980
  Start of program headers:          52 (bytes into file)
  Start of section headers:          18192664 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         8
  Size of section headers:           40 (bytes)
  Number of section headers:         28
  Section header string table index: 27
ben@australium:~/df_linux$ readelf -h /home/ben/df_linux/hack/libdfhack.so 
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 03 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2 complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - GNU
  ABI Version:                       0
  Type:                              DYN (Shared object file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x1b7e40
  Start of program headers:          52 (bytes into file)
  Start of section headers:          43686636 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         7
  Size of section headers:           40 (bytes)
  Number of section headers:         37
  Section header string table index: 34
ben@australium:~/df_linux$ readelf -h /usr/bin/gdb
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2 complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x808cb95
  Start of program headers:          52 (bytes into file)
  Start of section headers:          5089368 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         9
  Size of section headers:           40 (bytes)
  Number of section headers:         28
  Section header string table index: 27