diff --git a/beacon-chain/core/blocks/BUILD.bazel b/beacon-chain/core/blocks/BUILD.bazel
index 2af2845bc4cd..66a0c9c24848 100644
--- a/beacon-chain/core/blocks/BUILD.bazel
+++ b/beacon-chain/core/blocks/BUILD.bazel
@@ -42,6 +42,7 @@ go_test(
     srcs = [
         "block_operations_fuzz_test.go",
         "block_operations_test.go",
+        "block_regression_test.go",
         "block_test.go",
         "eth1_data_test.go",
     ],
diff --git a/beacon-chain/core/blocks/block_operations.go b/beacon-chain/core/blocks/block_operations.go
index 9c674d7cb835..a21cfce7c3f0 100644
--- a/beacon-chain/core/blocks/block_operations.go
+++ b/beacon-chain/core/blocks/block_operations.go
@@ -580,7 +580,7 @@ func slashableAttesterIndices(slashing *ethpb.AttesterSlashing) []uint64 {
 		return nil
 	}
 	indices1 := slashing.Attestation_1.AttestingIndices
-	indices2 := slashing.Attestation_1.AttestingIndices
+	indices2 := slashing.Attestation_2.AttestingIndices
 	return sliceutil.IntersectionUint64(indices1, indices2)
 }
 
diff --git a/beacon-chain/core/blocks/block_regression_test.go b/beacon-chain/core/blocks/block_regression_test.go
new file mode 100644
index 000000000000..ef7e94753ae0
--- /dev/null
+++ b/beacon-chain/core/blocks/block_regression_test.go
@@ -0,0 +1,111 @@
+package blocks_test
+
+import (
+	"context"
+	"testing"
+
+	ethpb "github.com/prysmaticlabs/ethereumapis/eth/v1alpha1"
+	"github.com/prysmaticlabs/prysm/beacon-chain/core/blocks"
+	"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
+	"github.com/prysmaticlabs/prysm/shared/bls"
+	"github.com/prysmaticlabs/prysm/shared/params"
+	"github.com/prysmaticlabs/prysm/shared/testutil"
+)
+
+func TestProcessAttesterSlashings_RegressionSlashableIndices(t *testing.T) {
+	testutil.ResetCache()
+	beaconState, privKeys := testutil.DeterministicGenesisState(t, 5500)
+	for _, vv := range beaconState.Validators() {
+		vv.WithdrawableEpoch = 1 * params.BeaconConfig().SlotsPerEpoch
+	}
+	// This set of indices is very similar to the one from our sapphire testnet
+	// when close to 100 validators were incorrectly slashed. The set is from 0 -5500,
+	// instead of 55000 as it would take too long to generate a state.
+	setA := []uint64{21, 92, 236, 244, 281, 321, 510, 524,
+		538, 682, 828, 858, 913, 920, 922, 959, 1176, 1207,
+		1222, 1229, 1354, 1394, 1436, 1454, 1510, 1550,
+		1552, 1576, 1645, 1704, 1842, 1967, 2076, 2111, 2134, 2307,
+		2343, 2354, 2417, 2524, 2532, 2555, 2740, 2749, 2759, 2762,
+		2800, 2809, 2824, 2987, 3110, 3125, 3559, 3583, 3599, 3608,
+		3657, 3685, 3723, 3756, 3759, 3761, 3820, 3826, 3979, 4030,
+		4141, 4170, 4205, 4247, 4257, 4479, 4492, 4569, 5091,
+	}
+	// Only 2800 is the slashable index.
+	setB := []uint64{1361, 1438, 2383, 2800}
+	expectedSlashedVal := 2800
+
+	root1 := [32]byte{'d', 'o', 'u', 'b', 'l', 'e', '1'}
+	att1 := &ethpb.IndexedAttestation{
+		Data: &ethpb.AttestationData{
+			Source: &ethpb.Checkpoint{Epoch: 0},
+			Target: &ethpb.Checkpoint{Epoch: 0, Root: root1[:]},
+		},
+		AttestingIndices: setA,
+	}
+	domain, err := helpers.Domain(beaconState.Fork(), 0, params.BeaconConfig().DomainBeaconAttester, beaconState.GenesisValidatorRoot())
+	if err != nil {
+		t.Fatal(err)
+	}
+	signingRoot, err := helpers.ComputeSigningRoot(att1.Data, domain)
+	if err != nil {
+		t.Errorf("Could not get signing root of beacon block header: %v", err)
+	}
+	aggSigs := []*bls.Signature{}
+	for _, index := range setA {
+		sig := privKeys[index].Sign(signingRoot[:])
+		aggSigs = append(aggSigs, sig)
+	}
+	aggregateSig := bls.AggregateSignatures(aggSigs)
+	att1.Signature = aggregateSig.Marshal()[:]
+
+	root2 := [32]byte{'d', 'o', 'u', 'b', 'l', 'e', '2'}
+	att2 := &ethpb.IndexedAttestation{
+		Data: &ethpb.AttestationData{
+			Source: &ethpb.Checkpoint{Epoch: 0},
+			Target: &ethpb.Checkpoint{Epoch: 0, Root: root2[:]},
+		},
+		AttestingIndices: setB,
+	}
+	signingRoot, err = helpers.ComputeSigningRoot(att2.Data, domain)
+	if err != nil {
+		t.Errorf("Could not get signing root of beacon block header: %v", err)
+	}
+	aggSigs = []*bls.Signature{}
+	for _, index := range setB {
+		sig := privKeys[index].Sign(signingRoot[:])
+		aggSigs = append(aggSigs, sig)
+	}
+	aggregateSig = bls.AggregateSignatures(aggSigs)
+	att2.Signature = aggregateSig.Marshal()[:]
+
+	slashings := []*ethpb.AttesterSlashing{
+		{
+			Attestation_1: att1,
+			Attestation_2: att2,
+		},
+	}
+
+	currentSlot := 2 * params.BeaconConfig().SlotsPerEpoch
+	beaconState.SetSlot(currentSlot)
+
+	block := &ethpb.BeaconBlock{
+		Body: &ethpb.BeaconBlockBody{
+			AttesterSlashings: slashings,
+		},
+	}
+
+	newState, err := blocks.ProcessAttesterSlashings(context.Background(), beaconState, block.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	newRegistry := newState.Validators()
+	if !newRegistry[expectedSlashedVal].Slashed {
+		t.Errorf("Validator with index %d was not slashed despite performing a double vote", expectedSlashedVal)
+	}
+
+	for idx, val := range newRegistry {
+		if val.Slashed && idx != expectedSlashedVal {
+			t.Errorf("validator with index: %d was unintentionally slashed", idx)
+		}
+	}
+}