diff --git a/src/lib/hooks/use-quiz.ts b/src/lib/hooks/use-quiz.ts
index 14e8471..362986b 100644
--- a/src/lib/hooks/use-quiz.ts
+++ b/src/lib/hooks/use-quiz.ts
@@ -12,7 +12,10 @@ export function useQuiz(courseId: string) {
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
-    if (!courseId) return;
+    if (!courseId) {
+      setLoading(false);
+      return;
+    }
     setLoading(true);
     getQuiz(courseId, jwt ?? undefined)
       .then(setQuiz)
diff --git a/src/lib/stellar/contracts.ts b/src/lib/stellar/contracts.ts
index 287c915..713c600 100644
--- a/src/lib/stellar/contracts.ts
+++ b/src/lib/stellar/contracts.ts
@@ -1,7 +1,7 @@
 import type { NetworkType } from "./wallet";
 import { simulateContractCall, signAndSubmitTransaction } from "./transactions";
 import type { TransactionResult } from "@/types/stellar";
-import { scval } from "@stellar/stellar-sdk";
+import { xdr, scValToNative } from "@stellar/stellar-sdk";
 
 // Validate required environment variables at startup
 function validateEnvironmentVariables() {
@@ -69,7 +69,10 @@ export async function readRewardBalance(
     throw new Error("Invalid simulation result");
   }
 
-  const resultArray = simResult.results || simResult;
+  const resultArray =
+    "results" in simResult && Array.isArray((simResult as Record<string, unknown>).results)
+      ? (simResult as Record<string, unknown>).results
+      : simResult;
   if (!Array.isArray(resultArray) || resultArray.length === 0) {
     throw new Error("No results in simulation response");
   }
@@ -79,7 +82,7 @@ export async function readRewardBalance(
     throw new Error("Missing XDR in simulation result");
   }
 
-  const scVal = scval.native.fromXDR(result.xdr, "base64");
+  const scVal = scValToNative(xdr.ScVal.fromXDR(result.xdr, "base64"));
   const balance = scVal as unknown as bigint | number | string;
   return String(balance);
 }
diff --git a/src/lib/stellar/transactions.ts b/src/lib/stellar/transactions.ts
index 95f8fe1..15169ba 100644
--- a/src/lib/stellar/transactions.ts
+++ b/src/lib/stellar/transactions.ts
@@ -1,5 +1,5 @@
 import freighterApi from "@stellar/freighter-api";
-import { Contract, Address, TransactionBuilder, xdr } from "@stellar/stellar-sdk";
+import { Contract, Account, TransactionBuilder, nativeToScVal } from "@stellar/stellar-sdk";
 import type { NetworkType } from "./wallet";
 import { getNetworkPassphrase, getRpcUrl } from "./wallet";
 import type { TransactionResult } from "@/types/stellar";
@@ -81,21 +81,14 @@ export async function simulateContractCall(
   const contract = new Contract(contractAddress);
 
   const txBuilder = new TransactionBuilder(
-    new Address(contractAddress),
+    new Account(contractAddress, "0"),
     {
       fee: "100",
       networkPassphrase: passphrase,
     }
   );
 
-  const sorobanArgs = args.map((arg) => {
-    if (typeof arg === "string") return xdr.ScVal.scvString(arg);
-    if (typeof arg === "number") return xdr.ScVal.scvU32(arg);
-    if (typeof arg === "bigint") return xdr.ScVal.scvU64(arg);
-    if (typeof arg === "boolean") return xdr.ScVal.scvBool(arg);
-    if (arg instanceof Uint8Array) return xdr.ScVal.scvBytes(arg);
-    return xdr.ScVal.scvString(JSON.stringify(arg));
-  });
+  const sorobanArgs = args.map((arg) => nativeToScVal(arg));
 
   const tx = txBuilder
     .addOperation(contract.call(method, ...sorobanArgs))