diff --git a/pkgs/development/lisp-modules/clwrapper/setup-hook.sh b/pkgs/development/lisp-modules/clwrapper/setup-hook.sh
index b48f916ac7a..bc5a6b34866 100644
--- a/pkgs/development/lisp-modules/clwrapper/setup-hook.sh
+++ b/pkgs/development/lisp-modules/clwrapper/setup-hook.sh
@@ -1,6 +1,6 @@
 NIX_LISP_ASDF="@asdf@"
 
-CL_SOURCE_REGISTRY="@asdf@/lib/common-lisp/asdf/:@asdf@/lib/common-lisp/asdf/uiop/"
+CL_SOURCE_REGISTRY="${CL_SOURCE_REGISTRY:+$CL_SOURCE_REGISTRY:}@asdf@/lib/common-lisp/asdf/:@asdf@/lib/common-lisp/asdf/uiop/"
 
 addASDFPaths () {
     for j in "$1"/lib/common-lisp/*; do