diff --git a/src/docset.rs b/src/docset.rs
index 24282f4a8..3c5dfdd31 100644
--- a/src/docset.rs
+++ b/src/docset.rs
@@ -10,7 +10,7 @@ use std::borrow::BorrowMut;
 pub const TERMINATED: DocId = std::i32::MAX as u32;
 
 /// Represents an iterable set of sorted doc ids.
-pub trait DocSet {
+pub trait DocSet: Send {
     /// Goes to the next element.
     ///
     /// The DocId of the next element is returned.
diff --git a/src/query/reqopt_scorer.rs b/src/query/reqopt_scorer.rs
index a9f26aa24..a3d39a928 100644
--- a/src/query/reqopt_scorer.rs
+++ b/src/query/reqopt_scorer.rs
@@ -12,7 +12,7 @@ use std::marker::PhantomData;
 /// This is useful for queries like `+somethingrequired somethingoptional`.
 ///
 /// Note that `somethingoptional` has no impact on the `DocSet`.
-pub struct RequiredOptionalScorer<TReqScorer, TOptScorer, TScoreCombiner> {
+pub struct RequiredOptionalScorer<TReqScorer, TOptScorer, TScoreCombiner: ScoreCombiner> {
     req_scorer: TReqScorer,
     opt_scorer: TOptScorer,
     score_cache: Option<Score>,
@@ -23,6 +23,7 @@ impl<TReqScorer, TOptScorer, TScoreCombiner>
     RequiredOptionalScorer<TReqScorer, TOptScorer, TScoreCombiner>
 where
     TOptScorer: DocSet,
+    TScoreCombiner: ScoreCombiner,
 {
     /// Creates a new `RequiredOptionalScorer`.
     pub fn new(
@@ -43,6 +44,7 @@ impl<TReqScorer, TOptScorer, TScoreCombiner> DocSet
 where
     TReqScorer: DocSet,
     TOptScorer: DocSet,
+    TScoreCombiner: ScoreCombiner,
 {
     fn advance(&mut self) -> DocId {
         self.score_cache = None;
diff --git a/src/query/score_combiner.rs b/src/query/score_combiner.rs
index 34be8a9e9..7af0e876a 100644
--- a/src/query/score_combiner.rs
+++ b/src/query/score_combiner.rs
@@ -3,7 +3,7 @@ use crate::Score;
 
 /// The `ScoreCombiner` trait defines how to compute
 /// an overall score given a list of scores.
-pub trait ScoreCombiner: Default + Clone + Copy + 'static {
+pub trait ScoreCombiner: Default + Clone + Send + Copy + 'static {
     /// Aggregates the score combiner with the given scorer.
     ///
     /// The `ScoreCombiner` may decide to call `.scorer.score()`