diff --git a/src/Text/Pandoc/Writers/Docx.hs b/src/Text/Pandoc/Writers/Docx.hs
index 30c604058..73ed095c1 100644
--- a/src/Text/Pandoc/Writers/Docx.hs
+++ b/src/Text/Pandoc/Writers/Docx.hs
@@ -95,6 +95,7 @@ instance Semigroup EnvProps where
 
 instance Monoid EnvProps where
   mempty = EnvProps Nothing []
+  mappend = (<>)
 
 squashProps :: EnvProps -> [Element]
 squashProps (EnvProps Nothing es) = es
diff --git a/src/Text/Pandoc/Writers/Muse.hs b/src/Text/Pandoc/Writers/Muse.hs
index 43f7f3b14..51a9ea7d6 100644
--- a/src/Text/Pandoc/Writers/Muse.hs
+++ b/src/Text/Pandoc/Writers/Muse.hs
@@ -629,7 +629,8 @@ inlineToMuse (Code _ str) = do
              then "<code>" <> text (substitute "</code>" "<</code><code>/code>" str) <> "</code>"
              else "=" <> text str <> "="
 inlineToMuse Math{} =
-  Prelude.fail "Math should be expanded before normalization"
+  throwError $ PandocShouldNeverHappenError
+    "Math should be expanded before normalization"
 inlineToMuse (RawInline (Format f) str) = do
   modify $ \st -> st { stUseTags = False }
   return $ "<literal style=\"" <> text f <> "\">" <> text str <> "</literal>"