diff --git a/beacon-chain/core/epoch/precompute/reward_penalty.go b/beacon-chain/core/epoch/precompute/reward_penalty.go
index 9a95364ba..07868da4f 100644
--- a/beacon-chain/core/epoch/precompute/reward_penalty.go
+++ b/beacon-chain/core/epoch/precompute/reward_penalty.go
@@ -162,6 +162,10 @@ func ProposersDelta(state *stateTrie.BeaconState, pBal *Balance, vp []*Validator
 	baseRewardsPerEpoch := params.BeaconConfig().BaseRewardsPerEpoch
 	proposerRewardQuotient := params.BeaconConfig().ProposerRewardQuotient
 	for _, v := range vp {
+		if v.ProposerIndex > uint64(len(rewards)) {
+			// This should never happen with a valid state / validator.
+			return nil, errors.New("proposer index out of range")
+		}
 		// Only apply inclusion rewards to proposer only if the attested hasn't been slashed.
 		if v.IsPrevEpochAttester && !v.IsSlashed {
 			vBalance := v.CurrentEpochEffectiveBalance
diff --git a/beacon-chain/core/epoch/precompute/reward_penalty_test.go b/beacon-chain/core/epoch/precompute/reward_penalty_test.go
index 085c3466f..3df1ddddb 100644
--- a/beacon-chain/core/epoch/precompute/reward_penalty_test.go
+++ b/beacon-chain/core/epoch/precompute/reward_penalty_test.go
@@ -359,6 +359,26 @@ func TestProposerDeltaPrecompute_HappyCase(t *testing.T) {
 	}
 }
 
+func TestProposerDeltaPrecompute_ValidatorIndexOutOfRange(t *testing.T) {
+	e := params.BeaconConfig().SlotsPerEpoch
+	validatorCount := uint64(10)
+	base := buildState(e, validatorCount)
+	state, err := state.InitializeFromProto(base)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	proposerIndex := validatorCount + 1
+	b := &Balance{ActiveCurrentEpoch: 1000}
+	v := []*Validator{
+		{IsPrevEpochAttester: true, CurrentEpochEffectiveBalance: 32, ProposerIndex: proposerIndex},
+	}
+	_, err = ProposersDelta(state, b, v)
+	if err == nil {
+		t.Fatal("Expected an error with invalid proposer index")
+	}
+}
+
 func TestProposerDeltaPrecompute_SlashedCase(t *testing.T) {
 	e := params.BeaconConfig().SlotsPerEpoch
 	validatorCount := uint64(10)