diff --git a/CHANGELOG.md b/CHANGELOG.md
index 49053f9..9578344 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,5 @@
+### Fixed
+- Error for empty `restClientConfig` while using HTTP retries.
 
 ## [5.5.7] - 2025-12-09
 ### Changed
diff --git a/__tests__/rest.spec.js b/__tests__/rest.spec.js
index ef70c95..7f6bb06 100644
--- a/__tests__/rest.spec.js
+++ b/__tests__/rest.spec.js
@@ -134,6 +134,26 @@ describe('RestClient', () => {
       };
       expect(retryConfig.retryCondition(timeoutError)).toBe(true);
     });
+
+    it('handles undefined restClientConfig without crashing during retries', () => {
+      const client = new RestClient({
+        baseURL: options.baseURL,
+        headers: options.headers,
+        restClientConfig: undefined,
+      });
+
+      const retryConfig = client.getRetryConfig();
+      expect(retryConfig.retries).toBe(6);
+
+      const consoleSpy = jest.spyOn(console, 'log').mockImplementation();
+      const onRetry = retryConfig.onRetry;
+      
+      expect(() => {
+        onRetry(1, { code: 'ECONNABORTED' }, { method: 'GET', url: 'http://test.com' });
+      }).not.toThrow();
+
+      consoleSpy.mockRestore();
+    });
   });
 
   describe('buildPath', () => {
diff --git a/lib/rest.js b/lib/rest.js
index e8ff4d9..5e3b002 100644
--- a/lib/rest.js
+++ b/lib/rest.js
@@ -150,7 +150,7 @@ method: ${method}`,
   getRetryConfig() {
     const retryOption = this.restClientConfig?.retry;
     const onRetry = (retryCount, error, requestConfig) => {
-      if (this.restClientConfig.debug) {
+      if (this.restClientConfig?.debug) {
         console.log(`[retry #${retryCount}] ${requestConfig.method?.toUpperCase()} ${requestConfig.url} -> ${error.code || error.message}`);
       }
     };