diff --git a/Nix/Eval.hs b/Nix/Eval.hs
index a1635ae..9d64f78 100644
--- a/Nix/Eval.hs
+++ b/Nix/Eval.hs
@@ -98,9 +98,10 @@ buildArgument paramSpec arg = either error id $ case paramSpec of
       let actualParamSet = case paramSet of
             FixedParamSet s -> s
             VariadicParamSet s -> s
-          maybeAddSet = case setName of
-            Just name -> Map.insert name arg
-            Nothing -> id
+          maybeAddSet attrsSet = case setName of
+            Just name -> let Fix (NVSet argSet) = arg
+                         in Map.insert name (Fix (NVSet (argSet `Map.union` attrsSet))) attrsSet
+            Nothing -> attrsSet
       in maybeAddSet <$> lookupParamSet actualParamSet
   where
     go env k def = maybe (Left err) return $ Map.lookup k env <|> def
diff --git a/tests/EvalTests.hs b/tests/EvalTests.hs
index 2765524..d7655ae 100644
--- a/tests/EvalTests.hs
+++ b/tests/EvalTests.hs
@@ -42,6 +42,9 @@ case_function_atpattern = constantEqualStr "2" "(({a}@attrs:attrs) {a=2;}).a"
 case_function_ellipsis :: Assertion
 case_function_ellipsis = constantEqualStr "2" "(({a, ...}@attrs:attrs) {a=0; b=2;}).b"
 
+case_function_default_value_in_atpattern :: Assertion
+case_function_default_value_in_atpattern = constantEqualStr "2" "({a ? 2}@attrs:attrs.a) {}"
+
 tests :: TestTree
 tests = $testGroupGenerator