diff --git a/CHANGES b/CHANGES
index 93377cd20..6962814e4 100644
--- a/CHANGES
+++ b/CHANGES
@@ -26,6 +26,7 @@ Bugs fixed
 * #4817: wrong URLs on warning messages
 * #4784: latex: :confval:`latex_show_urls` assigns incorrect footnote numbers if
   hyperlinks exists inside substitutions
+* #4837: latex: Class memoir Error Font command ``\sf`` is not supported
 
 Testing
 --------
diff --git a/sphinx/writers/latex.py b/sphinx/writers/latex.py
index 0fecd7b8a..aca5d91ab 100644
--- a/sphinx/writers/latex.py
+++ b/sphinx/writers/latex.py
@@ -615,7 +615,9 @@ class LaTeXTranslator(nodes.NodeVisitor):
         if builder.config.language \
            and 'fncychap' not in builder.config.latex_elements:
             # use Sonny style if any language specified
-            self.elements['fncychap'] = '\\usepackage[Sonny]{fncychap}'
+            self.elements['fncychap'] = ('\\usepackage[Sonny]{fncychap}'
+                                         '\\ChNameVar{\\Large\\sffamily}'
+                                         '\\ChTitleVar{\\Large\\sffamily}')
 
         self.babel = ExtBabel(builder.config.language)
         if builder.config.language and not self.babel.is_supported_language():