diff --git a/django/utils/html.py b/django/utils/html.py
index d914234d60..25605bea04 100644
--- a/django/utils/html.py
+++ b/django/utils/html.py
@@ -150,13 +150,17 @@ fix_ampersands = allow_lazy(fix_ampersands, six.text_type)
 def smart_urlquote(url):
     "Quotes a URL if it isn't already quoted."
     # Handle IDN before quoting.
-    scheme, netloc, path, query, fragment = urlsplit(url)
     try:
-        netloc = netloc.encode('idna').decode('ascii') # IDN -> ACE
-    except UnicodeError: # invalid domain part
+        scheme, netloc, path, query, fragment = urlsplit(url)
+        try:
+            netloc = netloc.encode('idna').decode('ascii') # IDN -> ACE
+        except UnicodeError: # invalid domain part
+            pass
+        else:
+            url = urlunsplit((scheme, netloc, path, query, fragment))
+    except ValueError:
+        # invalid IPv6 URL (normally square brackets in hostname part).
         pass
-    else:
-        url = urlunsplit((scheme, netloc, path, query, fragment))
 
     # An URL is considered unquoted if it contains no % characters or
     # contains a % not followed by two hexadecimal digits. See #9655.
diff --git a/tests/regressiontests/defaultfilters/tests.py b/tests/regressiontests/defaultfilters/tests.py
index 52268da2ec..8596f8c801 100644
--- a/tests/regressiontests/defaultfilters/tests.py
+++ b/tests/regressiontests/defaultfilters/tests.py
@@ -310,6 +310,10 @@ class DefaultFiltersTests(TestCase):
         self.assertEqual(urlize('[see www.example.com]'),
             '[see <a href="http://www.example.com" rel="nofollow">www.example.com</a>]' )
 
+        # Check urlize doesn't crash when square bracket is prepended to url (#19070)
+        self.assertEqual(urlize('see test[at[example.com'),
+            'see <a href="http://test[at[example.com" rel="nofollow">test[at[example.com</a>' )
+
 
     def test_wordcount(self):
         self.assertEqual(wordcount(''), 0)