diff --git a/nft-contract/src/nft_core.rs b/nft-contract/src/nft_core.rs
index 8616eb8..41e580f 100644
--- a/nft-contract/src/nft_core.rs
+++ b/nft-contract/src/nft_core.rs
@@ -13,7 +13,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: u64,
+        approval_id: Option<u64>,
         memo: Option<String>,
     );
 
@@ -24,7 +24,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: u64,
+        approval_id: Option<u64>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool>;
@@ -97,7 +97,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: u64,
+        approval_id: Option<u64>,
         memo: Option<String>,
     ) {
         //assert that the user attached exactly 1 yoctoNEAR. This is for security and so that the user will be redirected to the NEAR wallet. 
@@ -110,7 +110,7 @@ impl NonFungibleTokenCore for Contract {
             &sender_id,
             &receiver_id,
             &token_id,
-            Some(approval_id),
+            approval_id,
             memo,
         );
 
@@ -128,7 +128,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: u64,
+        approval_id: Option<u64>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool> {
@@ -158,7 +158,7 @@ impl NonFungibleTokenCore for Contract {
             &sender_id,
             &receiver_id,
             &token_id,
-            Some(approval_id),
+            approval_id,
             memo.clone(),
         );