diff --git a/testUpdateHostsFile.py b/testUpdateHostsFile.py
index 3dfa76f43..764402768 100644
--- a/testUpdateHostsFile.py
+++ b/testUpdateHostsFile.py
@@ -847,6 +847,7 @@ class TestNormalizeRule(BaseStdout):
             "0.0.0.0 https",
             "0.0.0.0 https..",
             "0.0.0.0 foo.",
+            "0.0.0.0 this_that.com",
         ]:
             self.assertEqual(normalize_rule(rule, **kwargs), (None, None))
 
diff --git a/updateHostsFile.py b/updateHostsFile.py
index 059cb731c..d57f36621 100755
--- a/updateHostsFile.py
+++ b/updateHostsFile.py
@@ -1133,6 +1133,7 @@ def normalize_rule(rule, target_ip, keep_domain_comments):
             or "." in hostname[-1]
             or "/" in hostname
             or ":" in hostname
+            or "_" in hostname
         ):
             # Example: 0.0.0.0 127.0.0.1
 
@@ -1143,6 +1144,7 @@ def normalize_rule(rule, target_ip, keep_domain_comments):
             #   - ends in a dot, or
             #   - contains a slash, or
             #   - contains a colon,
+            #   - contains an underscore,
             # we don't want to normalize it.
             return belch_unwanted(rule)