diff --git a/curve25519-dalek/src/backend/serial/u32/constants.rs b/curve25519-dalek/src/backend/serial/u32/constants.rs
index b5e6dcc..f55a822 100644
--- a/curve25519-dalek/src/backend/serial/u32/constants.rs
+++ b/curve25519-dalek/src/backend/serial/u32/constants.rs
@@ -69,7 +69,7 @@ pub(crate) const SQRT_M1: FieldElement2625 = FieldElement2625::from_limbs([
 
 /// `APLUS2_OVER_FOUR` is (A+2)/4. (This is used internally within the Montgomery ladder.)
 /// The u32e backend uses hardware acceleration for this.
-#[cfg(curve_dalek_backend != "u32e_backend")]
+#[cfg(not(curve_dalek_backend = "u32e_backend"))]
 pub(crate) const APLUS2_OVER_FOUR: FieldElement2625 =
     FieldElement2625::from_limbs([121666, 0, 0, 0, 0, 0, 0, 0, 0, 0]);