diff --git a/api.go b/api.go
index beb2741d0..c8aa2b5cc 100644
--- a/api.go
+++ b/api.go
@@ -741,13 +741,13 @@ func (api *API) ImportValue(_ context.Context, req *ImportValueRequest) error {
 }
 
 func importExistenceColumns(index *Index, columnIDs []uint64) error {
-	nnf := index.unprotectedExistenceField()
-	if nnf == nil {
+	ef := index.existenceField()
+	if ef == nil {
 		return nil
 	}
 
 	existenceRowIDs := make([]uint64, len(columnIDs))
-	return nnf.Import(existenceRowIDs, columnIDs, nil)
+	return ef.Import(existenceRowIDs, columnIDs, nil)
 }
 
 // MaxShards returns the maximum shard number for each index in a map.
diff --git a/executor.go b/executor.go
index 5681644aa..389cb573b 100644
--- a/executor.go
+++ b/executor.go
@@ -1132,8 +1132,8 @@ func (e *executor) executeSet(ctx context.Context, index string, c *pql.Call, op
 	}
 
 	// Set column on existence field.
-	if nnf := idx.unprotectedExistenceField(); nnf != nil {
-		if _, err := nnf.SetBit(0, colID, nil); err != nil {
+	if ef := idx.existenceField(); ef != nil {
+		if _, err := ef.SetBit(0, colID, nil); err != nil {
 			return false, errors.Wrap(err, "setting existence column")
 		}
 	}
diff --git a/index.go b/index.go
index 34ab01b28..0992ab213 100644
--- a/index.go
+++ b/index.go
@@ -36,9 +36,9 @@ type Index struct {
 	name string
 	keys bool // use string keys
 
-	// Not-null tracking.
+	// Existence tracking.
 	trackExistence bool
-	existenceField *Field
+	existenceFld   *Field
 
 	// Fields by name.
 	fields map[string]*Field
@@ -166,7 +166,7 @@ func (i *Index) openExistenceField() error {
 	if err != nil {
 		return errors.Wrap(err, "creating existence field")
 	}
-	i.existenceField = f
+	i.existenceFld = f
 	return nil
 }
 
@@ -275,9 +275,12 @@ func (i *Index) Fields() []*Field {
 	return a
 }
 
-// unprotectedExistenceField returns the internal field used to track column existence.
-func (i *Index) unprotectedExistenceField() *Field {
-	return i.existenceField
+// existenceField returns the internal field used to track column existence.
+func (i *Index) existenceField() *Field {
+	i.mu.RLock()
+	defer i.mu.RUnlock()
+
+	return i.existenceFld
 }
 
 // recalculateCaches recalculates caches on every field in the index.
@@ -425,7 +428,7 @@ func (i *Index) DeleteField(name string) error {
 	// turn off existence tracking on the index.
 	if name == existenceFieldName {
 		i.trackExistence = false
-		i.existenceField = nil
+		i.existenceFld = nil
 
 		// Update meta data on disk.
 		if err := i.saveMeta(); err != nil {
diff --git a/index_internal_test.go b/index_internal_test.go
index de4368d2a..fd83fd775 100644
--- a/index_internal_test.go
+++ b/index_internal_test.go
@@ -62,7 +62,7 @@ func TestIndex_Existence_Delete(t *testing.T) {
 		t.Fatalf("expected field to have been created: %s", existenceFieldName)
 	} else if !index.trackExistence {
 		t.Fatalf("expected index.trackExistence to be true")
-	} else if index.existenceField == nil {
+	} else if index.existenceFld == nil {
 		t.Fatalf("expected index.existenceField to be non-nil")
 	}
 
@@ -82,7 +82,7 @@ func TestIndex_Existence_Delete(t *testing.T) {
 		t.Fatalf("expected field to have been deleted: %s", existenceFieldName)
 	} else if index.trackExistence {
 		t.Fatalf("expected index.trackExistence to be false")
-	} else if index.existenceField != nil {
+	} else if index.existenceFld != nil {
 		t.Fatalf("expected index.existenceField to be nil")
 	}
 }