diff --git a/requests/packages/__init__.py b/requests/packages/__init__.py
index ec6a9e06..4dcf870f 100644
--- a/requests/packages/__init__.py
+++ b/requests/packages/__init__.py
@@ -27,9 +27,13 @@ import sys
 
 class VendorAlias(object):
 
-    def __init__(self):
+    def __init__(self, package_names):
+        self._package_names = package_names
         self._vendor_name = __name__
         self._vendor_pkg = self._vendor_name + "."
+        self._vendor_pkgs = [
+            self._vendor_pkg + name for name in self._package_names
+        ]
 
     def find_module(self, fullname, path=None):
         if fullname.startswith(self._vendor_pkg):
@@ -44,6 +48,14 @@ class VendorAlias(object):
                 )
             )
 
+        if not (name == self._vendor_name or
+                any(name.startswith(pkg) for pkg in self._vendor_pkgs)):
+            raise ImportError(
+                "Cannot import %s, must be one of %s." % (
+                    name, self._vendor_pkgs
+                )
+            )
+
         # Check to see if we already have this item in sys.modules, if we do
         # then simply return that.
         if name in sys.modules:
@@ -92,4 +104,4 @@ class VendorAlias(object):
         return module
 
 
-sys.meta_path.append(VendorAlias())
+sys.meta_path.append(VendorAlias(["urllib3", "chardet"]))
diff --git a/test_requests.py b/test_requests.py
index 69a10616..07430a8e 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -1611,5 +1611,12 @@ def test_urllib3_retries():
     with pytest.raises(RetryError):
         s.get(httpbin('status/500'))
 
+def test_vendor_aliases():
+    from requests.packages import urllib3
+    from requests.packages import chardet
+
+    with pytest.raises(ImportError):
+        from requests.packages import webbrowser
+
 if __name__ == '__main__':
     unittest.main()