diff --git a/src/Data/Syntax.hs b/src/Data/Syntax.hs
index 68a43b086..a20424f92 100644
--- a/src/Data/Syntax.hs
+++ b/src/Data/Syntax.hs
@@ -106,7 +106,7 @@ instance Ord1 Identifier where liftCompare = genericLiftCompare
 instance Show1 Identifier where liftShowsPrec = genericLiftShowsPrec
 
 instance Evaluatable Identifier where
-  eval (Identifier name) = lookupWith deref name >>= maybe (throwException $ FreeVariableError name) pure
+  eval (Identifier name) = lookupWith deref name >>= maybeM (throwException (FreeVariableError name))
 
 instance FreeVariables1 Identifier where
   liftFreeVariables _ (Identifier x) = pure x