diff --git a/requests/models.py b/requests/models.py
index cb44eddd..91555b58 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -347,9 +347,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         url = url.lstrip()
 
         # Don't do any URL preparation for non-HTTP schemes like `mailto`,
-        # `data`, `http+unix` etc to work around exceptions from `url_parse`,
-        # which handles RFC 3986 only.
-        if ':' in url and not url.lower().startswith(('http://', 'https://')):
+        # `data` etc to work around exceptions from `url_parse`, which
+        # handles RFC 3986 only.
+        if ':' in url and not url.lower().startswith('http'):
             self.url = url
             return
 
diff --git a/tests/test_requests.py b/tests/test_requests.py
index ca90297e..2c298bab 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -2177,18 +2177,72 @@ class TestPreparingURLs(object):
             r.prepare()
 
     @pytest.mark.parametrize(
-        'protocol, url',
+        'input, expected',
         (
-            ("http+unix://", b"http+unix://%2Fvar%2Frun%2Fsocket/path"),
-            ("http+unix://", u"http+unix://%2Fvar%2Frun%2Fsocket/path"),
-            ("mailto", b"mailto:user@example.org"),
-            ("mailto", u"mailto:user@example.org"),
-            ("data", b"data:SSDimaUgUHl0aG9uIQ=="),
+            (
+                b"http+unix://%2Fvar%2Frun%2Fsocket/path",
+                u"http+unix://%2fvar%2frun%2fsocket/path",
+            ),
+            (
+                u"http+unix://%2Fvar%2Frun%2Fsocket/path",
+                u"http+unix://%2fvar%2frun%2fsocket/path",
+            ),
+            (
+                b"mailto:user@example.org",
+                u"mailto:user@example.org",
+            ),
+            (
+                u"mailto:user@example.org",
+                u"mailto:user@example.org",
+            ),
+            (
+                b"data:SSDimaUgUHl0aG9uIQ==",
+                u"data:SSDimaUgUHl0aG9uIQ==",
+            )
         )
     )
-    def test_url_passthrough(self, protocol, url):
-        session = requests.Session()
-        session.mount(protocol, HTTPAdapter())
-        p = requests.Request('GET', url=url)
-        p.prepare()
-        assert p.url == url
+    def test_url_mutation(self, input, expected):
+        """
+        This test validates that we correctly exclude some URLs from
+        preparation, and that we handle others. Specifically, it tests that
+        any URL whose scheme doesn't begin with "http" is left alone, and
+        those whose scheme *does* begin with "http" are mutated.
+        """
+        r = requests.Request('GET', url=input)
+        p = r.prepare()
+        assert p.url == expected
+
+    @pytest.mark.parametrize(
+        'input, params, expected',
+        (
+            (
+                b"http+unix://%2Fvar%2Frun%2Fsocket/path",
+                {"key": "value"},
+                u"http+unix://%2fvar%2frun%2fsocket/path?key=value",
+            ),
+            (
+                u"http+unix://%2Fvar%2Frun%2Fsocket/path",
+                {"key": "value"},
+                u"http+unix://%2fvar%2frun%2fsocket/path?key=value",
+            ),
+            (
+                b"mailto:user@example.org",
+                {"key": "value"},
+                u"mailto:user@example.org",
+            ),
+            (
+                u"mailto:user@example.org",
+                {"key": "value"},
+                u"mailto:user@example.org",
+            ),
+        )
+    )
+    def test_parameters_for_nonstandard_schemes(self, input, params, expected):
+        """
+        Setting paramters for nonstandard schemes is allowed if those schemes
+        begin with "http", and is forbidden otherwise.
+        """
+        r = requests.Request('GET', url=input, params=params)
+        p = r.prepare()
+        assert p.url == expected
+