Memory map error for file input, not STDIN

[JeanMertz]

printf "# foo: bar\n" > test1
printf "# foo: bar" > test2

sd '# foo: \w+\n?' '' test1 test2
memory map must have a non-zero length
memory map must have a non-zero length

cat test1 | sd '# foo: \w+\n?' ''
cat test2 | sd '# foo: \w+\n?' ''

As seen in the above example, piping the contents of test1 and test2 to the sd command works as expected, whereas passing the files themselves results in an error.