diff --git a/pkgs/development/libraries/gtk/hooks/drop-icon-theme-cache.sh b/pkgs/development/libraries/gtk/hooks/drop-icon-theme-cache.sh
index f28a856c4f50..0762285843eb 100644
--- a/pkgs/development/libraries/gtk/hooks/drop-icon-theme-cache.sh
+++ b/pkgs/development/libraries/gtk/hooks/drop-icon-theme-cache.sh
@@ -16,4 +16,4 @@ dropIconThemeCache() {
     fi
 }
 
-preFixupPhases="${preFixupPhases-} dropIconThemeCache"
+appendToVar preFixupPhases dropIconThemeCache