diff --git a/CHANGES b/CHANGES
index bceb672d0..8b698ebf0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -38,7 +38,7 @@ Bugs fixed
 * #4067: Return non-zero exit status when make subprocess fails
 * #4055: graphviz: the :align: option does not work for SVG output
 * #4055: graphviz: the :align: center option does not work for latex output
-
+* #4051: ``warn()`` function for HTML theme outputs 'None' string
 
 
 Testing
diff --git a/sphinx/builders/html.py b/sphinx/builders/html.py
index 0265f6a1b..90e4574cc 100644
--- a/sphinx/builders/html.py
+++ b/sphinx/builders/html.py
@@ -898,7 +898,6 @@ class StandaloneHTMLBuilder(Builder):
                     outfilename=None, event_arg=None):
         # type: (unicode, Dict, unicode, unicode, Any) -> None
         ctx = self.globalcontext.copy()
-        ctx['warn'] = self.warn
         # current_page_name is backwards compatibility
         ctx['pagename'] = ctx['current_page_name'] = pagename
         ctx['encoding'] = self.config.html_output_encoding
@@ -931,6 +930,13 @@ class StandaloneHTMLBuilder(Builder):
             return False
         ctx['hasdoc'] = hasdoc
 
+        def warn(*args, **kwargs):
+            # type: (Any, Any) -> unicode
+            """Simple warn() wrapper for themes."""
+            self.warn(*args, **kwargs)
+            return ''  # return empty string
+        ctx['warn'] = warn
+
         ctx['toctree'] = lambda **kw: self._get_local_toctree(pagename, **kw)
         self.add_sidebars(pagename, ctx)
         ctx.update(addctx)