don't crash if fields is not a keyword #404

Job	Run time
static analysis (Elixir 1.15.x \| Erlang/OTP 26.x)	1m 15s
mix test (Elixir 1.13.x \| Erlang/OTP 22.x)	1m 5s
mix test windows (Elixir 1.13.x \| Erlang/OTP 22.x)	1m 56s
mix test (Elixir 1.13.x \| Erlang/OTP 23.x)	1m 9s
mix test windows (Elixir 1.13.x \| Erlang/OTP 23.x)	2m 12s
mix test (Elixir 1.13.x \| Erlang/OTP 24.x)	57s
mix test windows (Elixir 1.13.x \| Erlang/OTP 24.x)	1m 47s
mix test (Elixir 1.13.x \| Erlang/OTP 25.x)	58s
mix test windows (Elixir 1.13.x \| Erlang/OTP 25.x)	1m 47s
mix test (Elixir 1.14.x \| Erlang/OTP 23.x)	1m 1s
mix test windows (Elixir 1.14.x \| Erlang/OTP 23.x)	1m 53s
mix test (Elixir 1.14.x \| Erlang/OTP 24.x)	57s
mix test windows (Elixir 1.14.x \| Erlang/OTP 24.x)	1m 48s
mix test (Elixir 1.14.x \| Erlang/OTP 25.x)	1m 7s
mix test windows (Elixir 1.14.x \| Erlang/OTP 25.x)	2m 21s
mix test (Elixir 1.14.x \| Erlang/OTP 26.x)	1m 0s
mix test windows (Elixir 1.14.x \| Erlang/OTP 26.x)	2m 9s
mix test (Elixir 1.15.x \| Erlang/OTP 24.x)	58s
mix test (Elixir 1.15.x \| Erlang/OTP 25.x)	52s
mix test windows (Elixir 1.15.x \| Erlang/OTP 24.x)	1m 59s
mix test (Elixir 1.15.x \| Erlang/OTP 26.x)	51s
mix test windows (Elixir 1.15.x \| Erlang/OTP 25.x)	1m 37s
mix test windows (Elixir 1.15.x \| Erlang/OTP 26.x)	1m 37s
	33m 16s

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