diff --git a/src/models/easypost_object.js b/src/models/easypost_object.js
deleted file mode 100644
index deb10eb4..00000000
--- a/src/models/easypost_object.js
+++ /dev/null
@@ -1,27 +0,0 @@
-/**
- * An EasyPostObject is the base class for all EasyPost API resources.
- * @internal
- * @abstract
- */
-export default class EasyPostObject {
-  static id;
-  static object;
-  static mode;
-  static created_at;
-  static updated_at;
-  static _params;
-
-  static [Symbol.hasInstance](instance) {
-    if (instance == null || typeof instance !== 'object') {
-      return false;
-    }
-
-    const modelConstructor = instance[Symbol.for('easypost.modelConstructor')];
-    if (typeof modelConstructor === 'function') {
-      return modelConstructor === this;
-    }
-
-    // Fallback for plain API payloads that include object names like "Address" or "Shipment".
-    return typeof instance.object === 'string' && instance.object === this.name;
-  }
-}
diff --git a/src/models/easypost_object.ts b/src/models/easypost_object.ts
new file mode 100644
index 00000000..46b25e60
--- /dev/null
+++ b/src/models/easypost_object.ts
@@ -0,0 +1,32 @@
+/**
+ * An EasyPostObject is the base class for all EasyPost API resources.
+ * @internal
+ * @abstract
+ */
+export default class EasyPostObject {
+  static id: string;
+  static object: string;
+  static mode: string;
+  static created_at: string;
+  static updated_at: string;
+  static _params: Record<string, unknown>;
+
+  static [Symbol.hasInstance](instance: unknown): boolean {
+    if (instance == null || typeof instance !== 'object') {
+      return false;
+    }
+
+    const modelConstructor = (instance as Record<symbol, unknown>)[
+      Symbol.for('easypost.modelConstructor')
+    ];
+    if (typeof modelConstructor === 'function') {
+      return modelConstructor === this;
+    }
+
+    // Fallback for plain API payloads that include object names like "Address" or "Shipment".
+    return (
+      typeof (instance as Record<string, unknown>).object === 'string' &&
+      (instance as Record<string, unknown>).object === this.name
+    );
+  }
+}
diff --git a/src/services/base_service.js b/src/services/base_service.ts
similarity index 99%
rename from src/services/base_service.js
rename to src/services/base_service.ts
index 66aced01..10cd2d82 100644
--- a/src/services/base_service.js
+++ b/src/services/base_service.ts
@@ -207,7 +207,7 @@ export default (easypostClient) =>
      * @param {*} params The parameters passed when fetching the response.
      * @returns {*} A plain object or array suitable for JSON serialization.
      */
-    static _convertToEasyPostObject(response, params) {
+    static _convertToEasyPostObject(response, params = {}) {
       const modelResponse = this._buildEasyPostObject(response, params);
 
       return this._toPlainEasyPostObject(modelResponse);