diff --git a/database/src/key.rs b/database/src/key.rs
index a346e749..c0fee09d 100644
--- a/database/src/key.rs
+++ b/database/src/key.rs
@@ -69,7 +69,12 @@ macro_rules! impl_key {
             impl Key for $t {
                 const DUPLICATE: bool = false;
                 type Primary = $t;
-                type Secondary = $t;
+                // This 0 variant enum is unconstructable,
+                // and "has the same role as the ! “never” type":
+                // <https://doc.rust-lang.org/std/convert/enum.Infallible.html#future-compatibility>.
+                //
+                // FIXME: Use the `!` type when stable.
+                type Secondary = std::convert::Infallible;
 
                 #[inline(always)]
                 fn primary(self) -> Self::Primary {