diff --git a/src/Agda2Lambox/Compile/Function.hs b/src/Agda2Lambox/Compile/Function.hs
index 548d638..339454c 100644
--- a/src/Agda2Lambox/Compile/Function.hs
+++ b/src/Agda2Lambox/Compile/Function.hs
@@ -1,6 +1,6 @@
 {-# LANGUAGE NamedFieldPuns, DataKinds, OverloadedStrings #-}
 -- | Convert Agda functions to λ□ constant declarations
-module Agda2Lambox.Compile.Function 
+module Agda2Lambox.Compile.Function
   ( compileFunction
   ) where
 
@@ -106,7 +106,10 @@ compileFunction (t :: Target t) defn@Defn{defType} = do
 
     builder . flip LBox.LFix k <$>
       forM mdefs \def@Defn{defName} -> do
-        body <- compileFunctionBody mnames def
+        body <- compileFunctionBody mnames def >>= \case
+          l@LBox.LLambda{} -> pure l
+          LBox.LBox        -> pure $ LBox.LLambda LBox.Anon LBox.LBox
+          _                -> genericError "Fixpoint body must be Lambda."
         return LBox.Def
           { dName = qnameToName defName
           , dBody = body