diff --git a/related/inverted_index.go b/related/inverted_index.go
index fda6b9222..79dd4577c 100644
--- a/related/inverted_index.go
+++ b/related/inverted_index.go
@@ -274,9 +274,12 @@ func (cfg IndexConfig) ToKeywords(v interface{}) ([]Keyword, error) {
 		keywords = append(keywords, StringKeyword(vv))
 	case []string:
 		if toLower {
-			for i := 0; i < len(vv); i++ {
-				vv[i] = strings.ToLower(vv[i])
+			vc := make([]string, len(vv))
+			copy(vc, vv)
+			for i := 0; i < len(vc); i++ {
+				vc[i] = strings.ToLower(vc[i])
 			}
+			vv = vc
 		}
 		keywords = append(keywords, StringsToKeywords(vv...)...)
 	case time.Time:
diff --git a/related/inverted_index_test.go b/related/inverted_index_test.go
index 57e722364..576928aea 100644
--- a/related/inverted_index_test.go
+++ b/related/inverted_index_test.go
@@ -201,6 +201,21 @@ func TestSearch(t *testing.T) {
 
 }
 
+func TestToKeywordsToLower(t *testing.T) {
+	c := qt.New(t)
+	slice := []string{"A", "B", "C"}
+	config := IndexConfig{ToLower: true}
+	keywords, err := config.ToKeywords(slice)
+	c.Assert(err, qt.IsNil)
+	c.Assert(slice, qt.DeepEquals, []string{"A", "B", "C"})
+	c.Assert(keywords, qt.DeepEquals, []Keyword{
+		StringKeyword("a"),
+		StringKeyword("b"),
+		StringKeyword("c"),
+	})
+
+}
+
 func BenchmarkRelatedNewIndex(b *testing.B) {
 
 	pages := make([]*testDoc, 100)