diff --git a/helper/schema/set.go b/helper/schema/set.go
index 965ad6f8b8..a10c2b8d95 100644
--- a/helper/schema/set.go
+++ b/helper/schema/set.go
@@ -123,6 +123,9 @@ func (s *Set) add(item interface{}) int {
 	s.once.Do(s.init)
 
 	code := s.F(item)
+	if code < 0 {
+		code *= -1
+	}
 	if _, ok := s.m[code]; !ok {
 		s.m[code] = item
 	}
diff --git a/helper/schema/set_test.go b/helper/schema/set_test.go
index 8ae4be479f..9688c5f715 100644
--- a/helper/schema/set_test.go
+++ b/helper/schema/set_test.go
@@ -18,6 +18,20 @@ func TestSetAdd(t *testing.T) {
 	}
 }
 
+func TestSetAdd_negative(t *testing.T) {
+	// Since we don't allow negative hashes, this should just hash to the
+	// same thing...
+	s := &Set{F: testSetInt}
+	s.Add(-1)
+	s.Add(1)
+
+	expected := []interface{}{-1}
+	actual := s.List()
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
 func TestSetContains(t *testing.T) {
 	s := &Set{F: testSetInt}
 	s.Add(5)