diff --git a/pkgs/development/libraries/qt-5/hooks/qtbase-setup-hook.sh b/pkgs/development/libraries/qt-5/hooks/qtbase-setup-hook.sh
index 836e154148be..9f2a9f06f1ab 100644
--- a/pkgs/development/libraries/qt-5/hooks/qtbase-setup-hook.sh
+++ b/pkgs/development/libraries/qt-5/hooks/qtbase-setup-hook.sh
@@ -62,5 +62,5 @@ postPatchMkspecs() {
     fi
 }
 if [ -z "${dontPatchMkspecs-}" ]; then
-    postPhases="${postPhases}${postPhases:+ }postPatchMkspecs"
+    postPhases="${postPhases-}${postPhases:+ }postPatchMkspecs"
 fi