diff --git a/src/libexpr/fetchurl.nix b/src/libexpr/fetchurl.nix
index 38815fcc4..b487e959a 100644
--- a/src/libexpr/fetchurl.nix
+++ b/src/libexpr/fetchurl.nix
@@ -12,7 +12,7 @@
 , executable ? false
 , unpack ? false
 , name ? baseNameOf (toString url)
-, __impure ? false
+, impure ? false
 }:
 
 derivation ({