diff --git a/rest_framework_httpsignature/authentication.py b/rest_framework_httpsignature/authentication.py
index 5e31600..2b9f489 100644
--- a/rest_framework_httpsignature/authentication.py
+++ b/rest_framework_httpsignature/authentication.py
@@ -86,7 +86,11 @@ def authenticate(self, request):
         sent_string = request.META.get(authorization_header)
         if not sent_string:
             raise exceptions.AuthenticationFailed('No signature provided')
+
+        # Check if signature string matches signature pattern
         sent_signature = self.get_signature_from_signature_string(sent_string)
+        if not sent_signature:
+            return None
 
         # Fetch credentials for API key from the data store.
         try:
diff --git a/rest_framework_httpsignature/tests.py b/rest_framework_httpsignature/tests.py
index e2eda6e..06daa6a 100644
--- a/rest_framework_httpsignature/tests.py
+++ b/rest_framework_httpsignature/tests.py
@@ -177,11 +177,24 @@ def test_only_api_key(self):
         self.assertRaises(AuthenticationFailed,
                           self.auth.authenticate, request)
 
+    def test_no_signature(self):
+        request = RequestFactory().get(
+            ENDPOINT, {},
+            HTTP_X_API_KEY=KEYID,
+            HTTP_AUTHORIZATION='no-signature')
+        res = self.auth.authenticate(request)
+        self.assertIsNone(res)
+
     def test_bad_signature(self):
+        headers = ['(request-target)', 'accept', 'date', 'host']
+        signature = build_signature(
+            headers,
+            key_id=KEYID,
+            signature='some-wrong-value')
         request = RequestFactory().get(
             ENDPOINT, {},
             HTTP_X_API_KEY=KEYID,
-            HTTP_AUTHORIZATION='some-wrong-value')
+            HTTP_AUTHORIZATION=signature)
         self.assertRaises(AuthenticationFailed,
                           self.auth.authenticate, request)