GlyphCache.load_character fails with unwrapping None

[simendsjo]

line 78: self.data.get_mut(&size).unwrap()

thread '<main>' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:362
stack backtrace:
   1:     0x5556148d4a1e - sys::backtrace::write::h038220f50279f177vus
   2:     0x5556148d7de5 - panicking::on_panic::h71219c6695575a6bpgx
   3:     0x5556148cf5ae - rt::unwind::begin_unwind_inner::h82aba8beb356b6583Vw
   4:     0x5556148cfb6c - rt::unwind::begin_unwind_fmt::h39b470f67c9e92b29Uw
   5:     0x5556148d77d6 - rust_begin_unwind
   6:     0x55561490d564 - panicking::panic_fmt::hc9237b7ecab48700neC
   7:     0x55561490a1ed - panicking::panic::h1c877027618c7b18UcC
   8:     0x55561469f6b0 - option::Option<T>::unwrap::h16098042020249031139
                        at src/libcore/macros.rs:20
   9:     0x555614695c43 - glyph_cache::GlyphCache<'a>::load_character::hbf6ba35bcd92df86Lma
                        at /home/simendsjo/.cargo/registry/src/github.com-0a35038f75765ae4/piston2d-opengl_graphics-0.6.0/src/glyph_cache.rs:78
  10:     0x555614660169 - glyph_cache::GlyphCache<'a>::preload_chars::h2730168209668906689
                        at /home/simendsjo/.cargo/registry/src/github.com-0a35038f75765ae4/piston2d-opengl_graphics-0.6.0/src/glyph_cache.rs:101
  11:     0x55561465a4f3 - main::hdcc162e6c7822ac0Kia
                        at src/main.rs:62
  12:     0x5556148dcc48 - rust_try_inner
  13:     0x5556148dcc35 - rust_try
  14:     0x5556148d9918 - rt::lang_start::h725b432f757180fdmbx
  15:     0x55561466ca0b - main
  16:     0x7f132c67b78f - __libc_start_main
  17:     0x55561464ee38 - _start
  18:                0x0 - <unknown>
An unknown error occurred