diff --git a/parser/grammar.peg b/parser/grammar.peg
index 53a9947..50ebda7 100644
--- a/parser/grammar.peg
+++ b/parser/grammar.peg
@@ -132,7 +132,7 @@ Enum ← "enum" _ name:Identifier __ '{' __ values:(EnumValue __)* '}' EOS {
 	next := 0
 	for _, v := range vs {
 		ev := v.([]interface{})[0].(*EnumValue)
-		if ev.Value < 0 {
+		if !ev.Set {
 			ev.Value = next
 		}
 		if ev.Value >= next {
@@ -146,10 +146,10 @@ Enum ← "enum" _ name:Identifier __ '{' __ values:(EnumValue __)* '}' EOS {
 EnumValue ← name:Identifier _ value:('=' _ IntConstant)? ListSeparator? {
 	ev := &EnumValue{
 		Name: string(name.(Identifier)),
-		Value: -1,
 	}
 	if value != nil {
 		ev.Value = int(value.([]interface{})[2].(int64))
+		ev.Set = true
 	}
 	return ev, nil
 }
diff --git a/parser/parser_test.go b/parser/parser_test.go
index ba5cfdf..49610e6 100644
--- a/parser/parser_test.go
+++ b/parser/parser_test.go
@@ -191,10 +191,12 @@ func TestServiceParsing(t *testing.T) {
 			"ADD": &EnumValue{
 				Name:  "ADD",
 				Value: 1,
+				Set:   true,
 			},
 			"SUBTRACT": &EnumValue{
 				Name:  "SUBTRACT",
 				Value: 2,
+				Set:   true,
 			},
 		},
 	}
diff --git a/parser/types.go b/parser/types.go
index b9fc495..69dc08b 100644
--- a/parser/types.go
+++ b/parser/types.go
@@ -15,6 +15,7 @@ type Type struct {
 type EnumValue struct {
 	Name  string
 	Value int
+	Set   bool
 }
 
 type Enum struct {