diff --git a/int2.go b/int2.go
index 45bce93c..6156ea77 100644
--- a/int2.go
+++ b/int2.go
@@ -202,7 +202,7 @@ func (src *Int2) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus
diff --git a/int4.go b/int4.go
index a3499fef..37d00511 100644
--- a/int4.go
+++ b/int4.go
@@ -193,7 +193,7 @@ func (src *Int4) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus
diff --git a/int8.go b/int8.go
index d671eda7..17a676eb 100644
--- a/int8.go
+++ b/int8.go
@@ -179,7 +179,7 @@ func (src *Int8) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus
diff --git a/text.go b/text.go
index f05e1e89..e7fba682 100644
--- a/text.go
+++ b/text.go
@@ -144,7 +144,7 @@ func (src *Text) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus