diff --git a/src/parser.ts b/src/parser.ts
index 85cf768..6196dcc 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -41,8 +41,8 @@ function crawlChannelPropertiesForRefs(JSONSchema: AsyncAPIObject) {
  * @returns {boolean}
  * @private
  */
-function isExternalReference(ref: string) {
-  return !ref.startsWith('#');
+function isExternalReference(ref: string): boolean {
+  return typeof ref === 'string' && !ref.startsWith('#');
 }
 
 /**
diff --git a/tests/lib/index.spec.ts b/tests/lib/index.spec.ts
index 769ace5..b285340 100644
--- a/tests/lib/index.spec.ts
+++ b/tests/lib/index.spec.ts
@@ -40,4 +40,40 @@ describe('bundler should ', () => {
 
     expect(message.$ref).toMatch('#/components/messages/UserSignedUp');
   });
+
+  test('should not throw if value of `$ref` is not a string', async () => {
+    const files = ['./tests/wrong-ref-not-string.yaml'];
+
+    // If async function `bundle()` resolved Promise successfully, that means it
+    // did not throw exception during process of execution, which is the
+    // objective of testing.
+    expect(
+      await bundle(
+        files.map(file =>
+          fs.readFileSync(path.resolve(process.cwd(), file), 'utf-8')
+        ),
+        {
+          referenceIntoComponents: true,
+        }
+      )
+    ).resolves;
+  });
+
+  test('should not throw if value of `$ref` is absent', async () => {
+    const files = ['./tests/wrong-ref-absent.yaml'];
+
+    // If async function `bundle()` resolved Promise successfully, that means it
+    // did not throw exception during process of execution, which is the
+    // objective of testing.
+    expect(
+      await bundle(
+        files.map(file =>
+          fs.readFileSync(path.resolve(process.cwd(), file), 'utf-8')
+        ),
+        {
+          referenceIntoComponents: true,
+        }
+      )
+    ).resolves;
+  });
 });
diff --git a/tests/wrong-ref-absent.yaml b/tests/wrong-ref-absent.yaml
new file mode 100644
index 0000000..754ae6a
--- /dev/null
+++ b/tests/wrong-ref-absent.yaml
@@ -0,0 +1,10 @@
+asyncapi: '2.2.0'
+info:
+  title: Account Service
+  version: 1.0.0
+  description: This service is in charge of processing user signups
+channels:
+  user/signedup:
+    subscribe:
+      message:
+        $ref:
\ No newline at end of file
diff --git a/tests/wrong-ref-not-string.yaml b/tests/wrong-ref-not-string.yaml
new file mode 100644
index 0000000..e70e242
--- /dev/null
+++ b/tests/wrong-ref-not-string.yaml
@@ -0,0 +1,10 @@
+asyncapi: '2.2.0'
+info:
+  title: Account Service
+  version: 1.0.0
+  description: This service is in charge of processing user signups
+channels:
+  user/signedup:
+    subscribe:
+      message:
+        $ref: 5
\ No newline at end of file