diff --git a/pkgs/tools/networking/sipsak/default.nix b/pkgs/tools/networking/sipsak/default.nix
index 941e1d809ba1..852e80825b3d 100644
--- a/pkgs/tools/networking/sipsak/default.nix
+++ b/pkgs/tools/networking/sipsak/default.nix
@@ -10,6 +10,8 @@ stdenv.mkDerivation rec {
     c-ares
   ];
 
+  NIX_CFLAGS_COMPILE = "--std=gnu89";
+
   src = fetchurl {
     url = "https://github.com/sipwise/sipsak/archive/mr${version}.tar.gz";
     sha256 = "769fe59966b1962b67aa35aad7beb9a2110ebdface36558072a05c6405fb5374";