diff --git a/requests/adapters.py b/requests/adapters.py
index fd46325a..0d461b35 100644
--- a/requests/adapters.py
+++ b/requests/adapters.py
@@ -241,6 +241,7 @@ class HTTPAdapter(BaseAdapter):
                 conn.key_file = cert[1]
             else:
                 conn.cert_file = cert
+                conn.key_file = None
             if conn.cert_file and not os.path.exists(conn.cert_file):
                 raise IOError("Could not find the TLS certificate file, "
                               "invalid path: {0}".format(conn.cert_file))
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 0dad423c..d8a42151 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -786,6 +786,10 @@ class TestRequests:
             requests.get(httpbin_secure(), cert=('.', INVALID_PATH))
         assert str(e.value) == 'Could not find the TLS key file, invalid path: {0}'.format(INVALID_PATH)
 
+    def test_http_with_certificate(self, httpbin):
+        r = requests.get(httpbin(), cert='.')
+        assert r.status_code == 200
+
     def test_https_warnings(self, httpbin_secure, httpbin_ca_bundle):
         """warnings are emitted with requests.get"""
         if HAS_MODERN_SSL or HAS_PYOPENSSL: