Error message for E0307 is weird

[tbu-]

a.rs:8:17: 8:38 error: expected constant integer for repeat count, but unsupported constant expr [E0307]
a.rs:8     let a = [0; mem::size_of::<i32>()];
                       ^~~~~~~~~~~~~~~~~~~~~
a.rs:8:17: 8:38 help: run `rustc --explain E0307` to see a detailed explanation
error: aborting due to previous error

The message "expected constant integer for repeat count, but unsupported constant expr" is a bit weird (ungrammatical?).