diff --git a/src/tld/tests/test_core.py b/src/tld/tests/test_core.py
index d416e36..24c3b61 100644
--- a/src/tld/tests/test_core.py
+++ b/src/tld/tests/test_core.py
@@ -306,6 +306,15 @@ def setUp(self):
                 "tld": "com",
                 "kwargs": {"fail_silently": True},
             },
+            {
+                "url": "sftp://sftp.test.com",
+                "fld": "test.com",
+                "subdomain": "sftp",
+                "domain": "test",
+                "suffix": "com",
+                "tld": "com",
+                "kwargs": {"fail_silently": True, "fix_protocol": True},
+            },
         ]
 
         self.bad_patterns = {
diff --git a/src/tld/utils.py b/src/tld/utils.py
index e49794e..f7f1aea 100644
--- a/src/tld/utils.py
+++ b/src/tld/utils.py
@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import argparse
+import re
 import sys
 from codecs import open as codecs_open
 from functools import lru_cache
@@ -46,6 +47,7 @@
 )
 
 tld_names: Dict[str, Trie] = {}
+protocol_re = re.compile(r"^(?:[a-z0-9.+-:]*)//")
 
 
 def get_tld_names_container() -> Dict[str, Trie]:
@@ -306,7 +308,7 @@ def process_url(
     )
 
     if not isinstance(url, SplitResult):
-        if fix_protocol and not url.startswith(("//", "http://", "https://")):
+        if fix_protocol and not protocol_re.match(url):
             url = f"https://{url}"
 
         # Get parsed URL as we might need it later