diff --git a/pkgs/by-name/hy/hyperrogue/package.nix b/pkgs/by-name/hy/hyperrogue/package.nix
index e6b287554de2..cd0492506027 100644
--- a/pkgs/by-name/hy/hyperrogue/package.nix
+++ b/pkgs/by-name/hy/hyperrogue/package.nix
@@ -32,13 +32,13 @@ stdenv.mkDerivation (finalAttrs: {
     HYPERROGUE_USE_GLEW = 1;
     HYPERROGUE_USE_PNG = 1;
     HYPERROGUE_USE_ROGUEVIZ = 1;
-  };
 
-  CXXFLAGS = [
-    "-I${lib.getDev SDL}/include/SDL"
-    "-DHYPERPATH='\"${placeholder "out"}/share/hyperrogue/\"'"
-    "-DRESOURCEDESTDIR=HYPERPATH"
-  ];
+    CXXFLAGS = toString [
+      "-I${lib.getDev SDL}/include/SDL"
+      "-DHYPERPATH='\"${placeholder "out"}/share/hyperrogue/\"'"
+      "-DRESOURCEDESTDIR=HYPERPATH"
+    ];
+  };
 
   nativeBuildInputs = [
     pkg-config