diff --git a/src/fee_bump_transaction.js b/src/fee_bump_transaction.js
index ea91d8b1..42288072 100644
--- a/src/fee_bump_transaction.js
+++ b/src/fee_bump_transaction.js
@@ -46,7 +46,7 @@ export class FeeBumpTransaction extends TransactionBase {
     const innerTxEnvelope = xdr.TransactionEnvelope.envelopeTypeTx(
       tx.innerTx().v1()
     );
-    this._feeSource = encodeMuxedAccountToAddress(this.tx.feeSource());
+    this._feeSource = encodeMuxedAccountToAddress(this._tx.feeSource());
     this._innerTransaction = new Transaction(
       innerTxEnvelope,
       networkPassphrase
@@ -89,7 +89,7 @@ export class FeeBumpTransaction extends TransactionBase {
   signatureBase() {
     const taggedTransaction =
       new xdr.TransactionSignaturePayloadTaggedTransaction.envelopeTypeTxFeeBump(
-        this.tx
+        this._tx
       );
 
     const txSignature = new xdr.TransactionSignaturePayload({
@@ -106,7 +106,7 @@ export class FeeBumpTransaction extends TransactionBase {
    */
   toEnvelope() {
     const envelope = new xdr.FeeBumpTransactionEnvelope({
-      tx: xdr.FeeBumpTransaction.fromXDR(this.tx.toXDR()), // make a copy of the tx
+      tx: xdr.FeeBumpTransaction.fromXDR(this._tx.toXDR()), // make a copy of the tx
       signatures: this.signatures.slice() // make a copy of the signatures
     });
 
diff --git a/src/memo.js b/src/memo.js
index d9d0bfa5..84150072 100644
--- a/src/memo.js
+++ b/src/memo.js
@@ -102,6 +102,13 @@ export class Memo {
       throw error;
     }
 
+    // Reject anything that isn't plain decimal digits (no scientific notation,
+    // no decimals, no signs). BigNumber accepts formats like "1e18" and "1.0"
+    // which pass numeric checks but crash BigInt()/UnsignedHyper.fromString().
+    if (!/^\d+$/.test(value)) {
+      throw error;
+    }
+
     let number;
     try {
       number = new BigNumber(value);
@@ -109,26 +116,6 @@ export class Memo {
       throw error;
     }
 
-    // Infinity
-    if (!number.isFinite()) {
-      throw error;
-    }
-
-    // NaN
-    if (number.isNaN()) {
-      throw error;
-    }
-
-    // Negative
-    if (number.isNegative()) {
-      throw error;
-    }
-
-    // Decimal
-    if (!number.isInteger()) {
-      throw error;
-    }
-
     // Exceeds uint64 max (2^64 - 1)
     if (number.isGreaterThan('18446744073709551615')) {
       throw error;
diff --git a/src/transaction.js b/src/transaction.js
index 47d63255..4abec66a 100644
--- a/src/transaction.js
+++ b/src/transaction.js
@@ -62,11 +62,11 @@ export class Transaction extends TransactionBase {
     switch (this._envelopeType) {
       case xdr.EnvelopeType.envelopeTypeTxV0():
         this._source = StrKey.encodeEd25519PublicKey(
-          this.tx.sourceAccountEd25519()
+          this._tx.sourceAccountEd25519()
         );
         break;
       default:
-        this._source = encodeMuxedAccountToAddress(this.tx.sourceAccount());
+        this._source = encodeMuxedAccountToAddress(this._tx.sourceAccount());
         break;
     }
 
@@ -256,7 +256,7 @@ export class Transaction extends TransactionBase {
    * @returns {Buffer}
    */
   signatureBase() {
-    let tx = this.tx;
+    let tx = this._tx;
 
     // Backwards Compatibility: Use ENVELOPE_TYPE_TX to sign ENVELOPE_TYPE_TX_V0
     // we need a Transaction to generate the signature base
@@ -288,7 +288,7 @@ export class Transaction extends TransactionBase {
    * @returns {xdr.TransactionEnvelope}
    */
   toEnvelope() {
-    const rawTx = this.tx.toXDR();
+    const rawTx = this._tx.toXDR();
     const signatures = this.signatures.slice(); // make a copy of the signatures
 
     let envelope;
diff --git a/src/transaction_base.js b/src/transaction_base.js
index acda6af7..f92a3608 100644
--- a/src/transaction_base.js
+++ b/src/transaction_base.js
@@ -32,7 +32,11 @@ export class TransactionBase {
   }
 
   get tx() {
-    return this._tx;
+    // Return a defensive copy to prevent mutation of the internal XDR object
+    // after construction. Signing and serialization use this._tx directly,
+    // so exposing the live reference could let callers alter signed bytes
+    // without changing the cached display fields (fee, source, memo, etc.).
+    return this._tx.constructor.fromXDR(this._tx.toXDR());
   }
 
   set tx(value) {
diff --git a/test/unit/memo_test.js b/test/unit/memo_test.js
index dbb69816..549d4c92 100644
--- a/test/unit/memo_test.js
+++ b/test/unit/memo_test.js
@@ -134,6 +134,10 @@ describe('Memo.id()', function () {
     expect(() => StellarBase.Memo.id('-1')).to.throw(/Expects a uint64/);
     // decimal
     expect(() => StellarBase.Memo.id('1.5')).to.throw(/Expects a uint64/);
+    // trailing decimal zero (BigNumber accepts but must be rejected)
+    expect(() => StellarBase.Memo.id('1.0')).to.throw(/Expects a uint64/);
+    // scientific notation (BigNumber accepts but BigInt()/UnsignedHyper crashes)
+    expect(() => StellarBase.Memo.id('1e18')).to.throw(/Expects a uint64/);
     // overflow: 2^64 silently became 0 before this fix
     expect(() => StellarBase.Memo.id('18446744073709551616')).to.throw(
       /Expects a uint64/
diff --git a/test/unit/transaction_test.js b/test/unit/transaction_test.js
index 8e0e7d09..3cc0fbc7 100644
--- a/test/unit/transaction_test.js
+++ b/test/unit/transaction_test.js
@@ -79,6 +79,13 @@ describe('Transaction', function () {
       const envelope = transaction.toEnvelope().value();
       envelope.tx().fee(StellarBase.xdr.Int64.fromString('300'));
 
+      expect(transaction.tx.fee().toString()).to.equal('100');
+    });
+    it('tx getter returns a defensive copy that does not affect internal XDR', function () {
+      const transaction = this.transaction;
+      const txCopy = transaction.tx;
+      txCopy.fee(StellarBase.xdr.Int64.fromString('999'));
+
       expect(transaction.tx.fee().toString()).to.equal('100');
     });
   });