Don't try to read SLE with key 0 from the ledger:

* May resolve #4341

src/ripple/app/tx/impl/NFTokenAcceptOffer.cpp

@@ -67,6 +67,9 @@ NFTokenAcceptOffer::preclaim(PreclaimContext const& ctx)
         -> std::pair<std::shared_ptr<const SLE>, TER> {
         if (id)
         {
+            if (id->isZero())
+                return {nullptr, tecOBJECT_NOT_FOUND};
+
             auto offerSLE = ctx.view.read(keylet::nftoffer(*id));
 
             if (!offerSLE)

src/test/app/NFToken_test.cpp

@@ -1130,6 +1130,12 @@ class NFToken_test : public beast::unit_test::suite
         //----------------------------------------------------------------------
         // preclaim
 
+        // The buy offer must be non-zero.
+        env(token::acceptBuyOffer(buyer, beast::zero),
+            ter(tecOBJECT_NOT_FOUND));
+        env.close();
+        BEAST_EXPECT(ownerCount(env, buyer) == 0);
+
         // The buy offer must be present in the ledger.
         uint256 const missingOfferIndex = keylet::nftoffer(alice, 1).key;
         env(token::acceptBuyOffer(buyer, missingOfferIndex),
@@ -1142,6 +1148,12 @@ class NFToken_test : public beast::unit_test::suite
         env.close();
         BEAST_EXPECT(ownerCount(env, buyer) == 0);
 
+        // The sell offer must be non-zero.
+        env(token::acceptSellOffer(buyer, beast::zero),
+            ter(tecOBJECT_NOT_FOUND));
+        env.close();
+        BEAST_EXPECT(ownerCount(env, buyer) == 0);
+
         // The sell offer must be present in the ledger.
         env(token::acceptSellOffer(buyer, missingOfferIndex),
             ter(tecOBJECT_NOT_FOUND));