diff --git a/x/nft/keeper/grpc_query_test.go b/x/nft/keeper/grpc_query_test.go
index f70b0a013..0997721c4 100644
--- a/x/nft/keeper/grpc_query_test.go
+++ b/x/nft/keeper/grpc_query_test.go
@@ -57,7 +57,7 @@ func (suite *TestSuite) TestBalance() {
 				}
 			},
 			"",
-			1,
+			2,
 			func(index int, require *require.Assertions, res *nft.QueryBalanceResponse, expBalance uint64) {
 				require.Equal(res.Amount, expBalance, "the error occurred on:%d", index)
 			},
diff --git a/x/nft/keeper/keeper_test.go b/x/nft/keeper/keeper_test.go
index b6d17f1ae..150446349 100644
--- a/x/nft/keeper/keeper_test.go
+++ b/x/nft/keeper/keeper_test.go
@@ -158,6 +158,22 @@ func (s *TestSuite) TestMint() {
 	// test GetTotalSupply
 	supply := s.app.NFTKeeper.GetTotalSupply(s.ctx, testClassID)
 	s.Require().EqualValues(uint64(1), supply)
+
+	expNFT2 := nft.NFT{
+		ClassId: testClassID,
+		Id:      testID + "2",
+		Uri:     testURI + "2",
+	}
+	err = s.app.NFTKeeper.Mint(s.ctx, expNFT2, s.addrs[0])
+	s.Require().NoError(err)
+
+	// test GetNFTsOfClassByOwner
+	actNFTs = s.app.NFTKeeper.GetNFTsOfClassByOwner(s.ctx, testClassID, s.addrs[0])
+	s.Require().EqualValues([]nft.NFT{expNFT,expNFT2}, actNFTs)
+
+	// test GetBalance
+	balance = s.app.NFTKeeper.GetBalance(s.ctx, testClassID, s.addrs[0])
+	s.Require().EqualValues(uint64(2), balance)
 }
 
 func (s *TestSuite) TestBurn() {
diff --git a/x/nft/keeper/keys.go b/x/nft/keeper/keys.go
index bf8d1ceab..13c338089 100644
--- a/x/nft/keeper/keys.go
+++ b/x/nft/keeper/keys.go
@@ -53,7 +53,7 @@ func nftOfClassByOwnerStoreKey(owner sdk.AccAddress, classID string) []byte {
 	owner = address.MustLengthPrefix(owner)
 	classIDBz := conv.UnsafeStrToBytes(classID)
 
-	var key = make([]byte, len(NFTOfClassByOwnerKey)+len(owner)+len(Delimiter))
+	var key = make([]byte, len(NFTOfClassByOwnerKey)+len(owner)+len(classIDBz)+len(Delimiter))
 	copy(key, NFTOfClassByOwnerKey)
 	copy(key[len(NFTOfClassByOwnerKey):], owner)
 	copy(key[len(NFTOfClassByOwnerKey)+len(owner):], classIDBz)