diff --git a/scrapy/utils/url.py b/scrapy/utils/url.py
index c0934ddcf..398407a64 100644
--- a/scrapy/utils/url.py
+++ b/scrapy/utils/url.py
@@ -111,11 +111,13 @@ def escape_ajax(url):
         return url
     return add_or_replace_parameter(defrag, '_escaped_fragment_', frag[1:])
 
+
 def add_http_if_no_scheme(url):
-    """Adds http as the default scheme if it is missing from the url"""
-    parser = parse_url(url)
+    """Add http as the default scheme if it is missing from the url."""
     if url.startswith('//'):
         url = 'http:' + url
-    elif not parser.scheme or not parser.netloc:
+        return url
+    parser = parse_url(url)
+    if not parser.scheme or not parser.netloc:
         url = 'http://' + url
     return url