formatting fractional seconds besides 0/3/etc crashes

[donpdonp]

chrono 0.4.11 rustc 1.42.0
now.format("%FT%T%.3f").to_string() => "2020-05-05T12:57:42.506"

I was looking for just 1 decimal place in the fractional seconds.

now.format("%FT%T%.1f").to_string() => 
 12: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  13: core::option::expect_none_failed
             at src/libcore/option.rs:1199
  14: core::result::Result<T,E>::expect
             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libcore/result.rs:991
  15: <T as alloc::string::ToString>::to_string
             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/liballoc/s