diff --git a/pkgs/development/libraries/libgda/6.x.nix b/pkgs/development/libraries/libgda/6.x.nix
index af05b23080eb..72f4741fd2e7 100644
--- a/pkgs/development/libraries/libgda/6.x.nix
+++ b/pkgs/development/libraries/libgda/6.x.nix
@@ -85,7 +85,7 @@ stdenv.mkDerivation rec {
     postgresql
   ];
 
-  env.NIX_CFLAGS_COMPILE = lib.optionals stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
+  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
 
   postPatch = ''
     patchShebangs \
diff --git a/pkgs/development/libraries/libgda/default.nix b/pkgs/development/libraries/libgda/default.nix
index aa136ed0adbd..a8af4d9b4874 100644
--- a/pkgs/development/libraries/libgda/default.nix
+++ b/pkgs/development/libraries/libgda/default.nix
@@ -82,7 +82,7 @@ stdenv.mkDerivation rec {
     "--enable-system-sqlite=${if stdenv.isDarwin then "no" else "yes"}"
   ];
 
-  env.NIX_CFLAGS_COMPILE = lib.optionals stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
+  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=incompatible-function-pointer-types";
 
   enableParallelBuilding = true;