protect against json stringify

src/connection.ts

@@ -4557,19 +4557,23 @@ export class Connection {
       context,
       value: {blockhash},
     } = await this.getLatestBlockhashAndContext(commitment);
+    const feeCalculator = {
+      get lamportsPerSignature(): number {
+        throw new Error(
+          'The capability to fetch `lamportsPerSignature` using the `getRecentBlockhash` API is ' +
+            'no longer offered by the network. Use the `getFeeForMessage` API to obtain the fee ' +
+            'for a given message.',
+        );
+      },
+      toJSON() {
+        return {};
+      },
+    };
     return {
       context,
       value: {
         blockhash,
-        feeCalculator: {
-          get lamportsPerSignature(): number {
-            throw new Error(
-              'The capability to fetch `lamportsPerSignature` using the `getRecentBlockhash` API is ' +
-                'no longer offered by the network. Use the `getFeeForMessage` API to obtain the fee ' +
-                'for a given message.',
-            );
-          },
-        },
+        feeCalculator,
       },
     };
   }

test/connection.test.ts

@@ -4625,6 +4625,19 @@ describe('Connection', function () {
     }
   });
 
+  it('get recent blockhash can stringify', async () => {
+    const commitments: Commitment[] = ['processed', 'confirmed', 'finalized'];
+    for (const commitment of commitments) {
+      const response = await helpers.recentBlockhash({
+        connection,
+        commitment,
+      });
+      expect(JSON.stringify(response)).to.match(
+        /{"blockhash":"\w+","feeCalculator":{}}/,
+      );
+    }
+  });
+
   it('get recent blockhash LPS field throws', async () => {
     const commitments: Commitment[] = ['processed', 'confirmed', 'finalized'];
     for (const commitment of commitments) {