diff --git a/pkgs/kde/misc/phonon/default.nix b/pkgs/kde/misc/phonon/default.nix
index 76e57f30b3a5..a822d0e112ab 100644
--- a/pkgs/kde/misc/phonon/default.nix
+++ b/pkgs/kde/misc/phonon/default.nix
@@ -33,6 +33,8 @@ mkKdeDerivation rec {
   cmakeFlags = [
     "-DPHONON_BUILD_QT5=0"
     "-DPHONON_BUILD_QT6=1"
+    # Do not expose upstream's unstable experimental API.
+    (lib.cmakeBool "PHONON_BUILD_EXPERIMENTAL" false)
   ];
 
   meta.license = with lib.licenses; [