proc-macro derive produced unparseable tokens

[Dirbaio]

#[derive(defmt::Format)]
pub struct Set<'a, 'b: 'a> {
    foo: &'a u32,
    bar: &'b u32,
}

produces this error

error: expected one of `,` or `>`, found `:`
   --> foobar/src/lib.rs:140:22
    |
140 | pub struct Set<'a, 'b: 'a> {
    |                      ^ expected one of `,` or `>`

error: proc-macro derive produced unparseable tokens
   --> foobar/src/lib.rs:139:10
    |
139 | #[derive(defmt::Format)]
    |          ^^^^^^^^^^^^^

defmt version cf066cb
rustc 1.48.0-nightly (6af1bdda5 2020-09-15)