diff --git a/CHANGES b/CHANGES
index 1cd9f82b6..8ac344859 100644
--- a/CHANGES
+++ b/CHANGES
@@ -25,6 +25,7 @@ Bugs fixed
 * #3835: sphinx.ext.imgmath fails to convert SVG images if project directory
   name contains spaces
 * #3850: Fix color handling in make mode's help command
+* #3865: use of self.env.warn in sphinx extension fails
 
 Testing
 --------
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 1c47b4b3e..ab6414f3b 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -300,8 +300,7 @@ class BuildEnvironment(object):
         be emitted instantly, but collected for emitting all warnings after
         the update of the environment.
         """
-        # strange argument order is due to backwards compatibility
-        self._warnfunc(msg, (docname, lineno), **kwargs)
+        self.app.warn(msg, location=(docname, lineno), **kwargs)
 
     def warn_node(self, msg, node, **kwargs):
         # type: (unicode, nodes.Node, Any) -> None