diff --git a/bellman/src/groth16/tests/dummy_engine.rs b/bellman/src/groth16/tests/dummy_engine.rs
index 4692078..b25e713 100644
--- a/bellman/src/groth16/tests/dummy_engine.rs
+++ b/bellman/src/groth16/tests/dummy_engine.rs
@@ -22,7 +22,7 @@ impl fmt::Display for Fr {
 }
 
 impl Field for Fr {
-    fn random<R: RngCore>(rng: &mut R) -> Self {
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
         Fr(Wrapping(rng.next_u32()) % MODULUS_R)
     }
 
diff --git a/zcash_primitives/src/jubjub/fs.rs b/zcash_primitives/src/jubjub/fs.rs
index 7cf4d79..81120e6 100644
--- a/zcash_primitives/src/jubjub/fs.rs
+++ b/zcash_primitives/src/jubjub/fs.rs
@@ -317,7 +317,7 @@ impl PrimeField for Fs {
 }
 
 impl Field for Fs {
-    fn random<R: RngCore>(rng: &mut R) -> Self {
+    fn random<R: RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
         loop {
             let mut tmp = {
                 let mut repr = [0u64; 4];