Compiler assert when using _ as an argument to Array

[dipinhora]

In the following code:

actor Main
  new create(env:Env) =>
    let a = Array[U8]
    a.push(_)

The last line causes the following assert in the compiler:

vagrant@buffy-leader-1:~/ponyc$ ./build/debug/ponyc -d z
Building builtin -> /home/vagrant/ponyc/packages/builtin
Building z -> /home/vagrant/ponyc/z
src/libponyc/type/safeto.c:51: safe_field_write: Assertion `0` failed.

Backtrace:
  ./build/debug/ponyc(ponyint_assert_fail+0xe9) [0x71da60]
  ./build/debug/ponyc() [0x712f1d]
  ./build/debug/ponyc(safe_to_autorecover+0x123) [0x7132af]
  ./build/debug/ponyc() [0x6f6c0a]
  ./build/debug/ponyc() [0x6f6e92]
  ./build/debug/ponyc() [0x6f7675]
  ./build/debug/ponyc() [0x6f77d0]
  ./build/debug/ponyc(expr_call+0xf4) [0x6fa867]
  ./build/debug/ponyc(pass_expr+0x223) [0x66f3c1]
  ./build/debug/ponyc(ast_visit+0x261) [0x66d7c4]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc(ast_visit+0x1b3) [0x66d716]
  ./build/debug/ponyc() [0x66ce8a]
  ./build/debug/ponyc() [0x66d227]
  ./build/debug/ponyc(ast_passes_program+0x28) [0x66d354]
  ./build/debug/ponyc(program_load+0xbf) [0x668fc2]
  ./build/debug/ponyc() [0x649cbd]
  ./build/debug/ponyc(main+0x4cd) [0x64a207]
  /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7fa3fcfb8830]
  ./build/debug/ponyc(_start+0x29) [0x649b09]
Aborted (core dumped)

This same error also occurs if _ is passed as an argument to any function except for a constructor of a class where I get an error that says can't read from '_'.