diff --git a/ff_derive/src/lib.rs b/ff_derive/src/lib.rs
index 7cdaf91ed..5e56e7499 100644
--- a/ff_derive/src/lib.rs
+++ b/ff_derive/src/lib.rs
@@ -883,7 +883,7 @@ fn prime_field_impl(
 
         impl ::ff::Field for #name {
             /// Computes a uniformly random element using rejection sampling.
-            fn random<R: ::rand_core::RngCore>(rng: &mut R) -> Self {
+            fn random<R: ::rand_core::RngCore + ?std::marker::Sized>(rng: &mut R) -> Self {
                 loop {
                     let mut tmp = {
                         let mut repr = [0u64; #limbs];
diff --git a/src/lib.rs b/src/lib.rs
index 6be25dab5..c66163db4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -21,7 +21,7 @@ pub trait Field:
     Sized + Eq + Copy + Clone + Send + Sync + fmt::Debug + fmt::Display + 'static
 {
     /// 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 zero element of the field, the additive identity.
     fn zero() -> Self;