diff --git a/bellman/src/groth16/tests/dummy_engine.rs b/bellman/src/groth16/tests/dummy_engine.rs
index b25e713..7311545 100644
--- a/bellman/src/groth16/tests/dummy_engine.rs
+++ b/bellman/src/groth16/tests/dummy_engine.rs
@@ -299,7 +299,7 @@ impl CurveProjective for Fr {
     type Scalar = Fr;
     type Engine = DummyEngine;
 
-    fn random<R: RngCore>(rng: &mut R) -> Self {
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
         <Fr as Field>::random(rng)
     }
 
diff --git a/group/src/lib.rs b/group/src/lib.rs
index be78b2a..1c5d03e 100644
--- a/group/src/lib.rs
+++ b/group/src/lib.rs
@@ -22,7 +22,7 @@ pub trait CurveProjective:
     type Affine: CurveAffine<Projective = Self, Scalar = Self::Scalar>;
 
     /// Returns an element chosen uniformly at random using a user-provided RNG.
-    fn random<R: RngCore>(rng: &mut R) -> Self;
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self;
 
     /// Returns the additive identity.
     fn zero() -> Self;