diff --git a/pkgs/by-name/sp/spdlog/package.nix b/pkgs/by-name/sp/spdlog/package.nix
index 6aa91db83549..0c9777b1ca88 100644
--- a/pkgs/by-name/sp/spdlog/package.nix
+++ b/pkgs/by-name/sp/spdlog/package.nix
@@ -41,6 +41,11 @@ stdenv.mkDerivation rec {
   postInstall = ''
     mkdir -p $out/share/doc/spdlog
     cp -rv ../example $out/share/doc/spdlog
+
+    substituteInPlace $dev/include/spdlog/tweakme.h \
+      --replace-fail \
+        '// #define SPDLOG_FMT_EXTERNAL' \
+        '#define SPDLOG_FMT_EXTERNAL'
   '';
 
   doCheck = true;