diff --git a/codec_generic_internal_test.go b/codec_generic_internal_test.go
index 98a5855..a55f26b 100644
--- a/codec_generic_internal_test.go
+++ b/codec_generic_internal_test.go
@@ -173,6 +173,14 @@ func TestGenericDecode(t *testing.T) {
 			want:    map[string]any{"string": "foo"},
 			wantErr: require.NoError,
 		},
+		{
+			name: "Union Zero Index",
+			// 0x80 represents 128. So the bytes below will result in 0
+			// as a result of zig-zag encoding.
+			data:    []byte{0x80, 0x80, 0x80, 0x80, 0x30},
+			schema:  `["null"]`,
+			wantErr: require.NoError,
+		},
 		{
 			name:    "Union Nil",
 			data:    []byte{0x00},
diff --git a/codec_union.go b/codec_union.go
index 9e3649a..b6fd192 100644
--- a/codec_union.go
+++ b/codec_union.go
@@ -152,7 +152,7 @@ func (e *mapUnionEncoder) Encode(ptr unsafe.Pointer, w *Writer) {
 		return
 	}
 
-	w.WriteLong(int64(pos))
+	w.WriteInt(int32(pos))
 
 	if schema.Type() == Null && val == nil {
 		return
@@ -259,8 +259,8 @@ func encoderOfNullableUnion(cfg *frozenConfig, schema Schema, typ reflect2.Type)
 		schema:  union,
 		encoder: encoder,
 		isPtr:   isPtr,
-		nullIdx: int64(nullIdx),
-		typeIdx: int64(typeIdx),
+		nullIdx: int32(nullIdx),
+		typeIdx: int32(typeIdx),
 	}
 }
 
@@ -268,17 +268,17 @@ type unionNullableEncoder struct {
 	schema  *UnionSchema
 	encoder ValEncoder
 	isPtr   bool
-	nullIdx int64
-	typeIdx int64
+	nullIdx int32
+	typeIdx int32
 }
 
 func (e *unionNullableEncoder) Encode(ptr unsafe.Pointer, w *Writer) {
 	if *((*unsafe.Pointer)(ptr)) == nil {
-		w.WriteLong(e.nullIdx)
+		w.WriteInt(e.nullIdx)
 		return
 	}
 
-	w.WriteLong(e.typeIdx)
+	w.WriteInt(e.typeIdx)
 	newPtr := ptr
 	if e.isPtr {
 		newPtr = *((*unsafe.Pointer)(ptr))
@@ -445,7 +445,7 @@ type unionResolverEncoder struct {
 }
 
 func (e *unionResolverEncoder) Encode(ptr unsafe.Pointer, w *Writer) {
-	w.WriteLong(int64(e.pos))
+	w.WriteInt(int32(e.pos))
 
 	e.encoder.Encode(ptr, w)
 }
@@ -453,7 +453,7 @@ func (e *unionResolverEncoder) Encode(ptr unsafe.Pointer, w *Writer) {
 func getUnionSchema(schema *UnionSchema, r *Reader) (int, Schema) {
 	types := schema.Types()
 
-	idx := int(r.ReadLong())
+	idx := int(r.ReadInt())
 	if idx < 0 || idx > len(types)-1 {
 		r.ReportError("decode union type", "unknown union type")
 		return 0, nil