diff --git a/src/mdurl/_parse.py b/src/mdurl/_parse.py
index 9eb57a7..ffeeac7 100644
--- a/src/mdurl/_parse.py
+++ b/src/mdurl/_parse.py
@@ -202,7 +202,7 @@ def parse(self, url: str, slashes_denote_host: bool) -> "MutableURL":
             if host_end == -1:
                 host_end = len(rest)
 
-            if rest[host_end - 1] == ":":
+            if host_end > 0 and rest[host_end - 1] == ":":
                 host_end -= 1
             host = rest[:host_end]
             rest = rest[host_end:]
diff --git a/tests/fixtures/url.py b/tests/fixtures/url.py
index 7dc311c..0bae987 100644
--- a/tests/fixtures/url.py
+++ b/tests/fixtures/url.py
@@ -601,4 +601,10 @@
         "pathname": "/fooA100%mBr",
         "slashes": True,
     },
+    #
+    "http://": {
+        "protocol": "http:",
+        "hostname": "",
+        "slashes": True,
+    }
 }