diff --git a/helper/schema/set.go b/helper/schema/set.go
index 689be4f9f..78c68bdb7 100644
--- a/helper/schema/set.go
+++ b/helper/schema/set.go
@@ -59,7 +59,7 @@ func (s *Set) List() []interface{} {
 // a new third set that has only the elements unique to this set.
 func (s *Set) Difference(other *Set) *Set {
 	result := &Set{F: s.F}
-	result.init()
+	result.once.Do(result.init)
 
 	for k, v := range s.m {
 		if _, ok := other.m[k]; !ok {
@@ -74,7 +74,7 @@ func (s *Set) Difference(other *Set) *Set {
 // and returns a new third set.
 func (s *Set) Intersection(other *Set) *Set {
 	result := &Set{F: s.F}
-	result.init()
+	result.once.Do(result.init)
 
 	for k, v := range s.m {
 		if _, ok := other.m[k]; ok {
@@ -89,7 +89,7 @@ func (s *Set) Intersection(other *Set) *Set {
 // set.
 func (s *Set) Union(other *Set) *Set {
 	result := &Set{F: s.F}
-	result.init()
+	result.once.Do(result.init)
 
 	for k, v := range s.m {
 		result.m[k] = v
diff --git a/helper/schema/set_test.go b/helper/schema/set_test.go
index 7c9caa96e..8ae4be479 100644
--- a/helper/schema/set_test.go
+++ b/helper/schema/set_test.go
@@ -40,8 +40,11 @@ func TestSetDifference(t *testing.T) {
 	s2.Add(5)
 	s2.Add(25)
 
-	expected := []interface{}{1}
-	actual := s1.Difference(s2).List()
+	difference := s1.Difference(s2)
+	difference.Add(2)
+
+	expected := []interface{}{1, 2}
+	actual := difference.List()
 	if !reflect.DeepEqual(actual, expected) {
 		t.Fatalf("bad: %#v", actual)
 	}
@@ -57,8 +60,11 @@ func TestSetIntersection(t *testing.T) {
 	s2.Add(5)
 	s2.Add(25)
 
-	expected := []interface{}{5}
-	actual := s1.Intersection(s2).List()
+	intersection := s1.Intersection(s2)
+	intersection.Add(2)
+
+	expected := []interface{}{2, 5}
+	actual := intersection.List()
 	if !reflect.DeepEqual(actual, expected) {
 		t.Fatalf("bad: %#v", actual)
 	}
@@ -74,8 +80,11 @@ func TestSetUnion(t *testing.T) {
 	s2.Add(5)
 	s2.Add(25)
 
-	expected := []interface{}{1, 5, 25}
-	actual := s1.Union(s2).List()
+	union := s1.Union(s2)
+	union.Add(2)
+
+	expected := []interface{}{1, 2, 5, 25}
+	actual := union.List()
 	if !reflect.DeepEqual(actual, expected) {
 		t.Fatalf("bad: %#v", actual)
 	}