diff --git a/third_party/nix/CMakeLists.txt b/third_party/nix/CMakeLists.txt
index 200b604c2..5d89572f1 100644
--- a/third_party/nix/CMakeLists.txt
+++ b/third_party/nix/CMakeLists.txt
@@ -41,7 +41,8 @@ if (CLANG_TIDY_PATH)
 endif()
 
 if (DEFINED ENV{SANDBOX_SHELL})
-  set(SANDBOX_SHELL ENV{SANDBOX_SHELL})
+  message("Using SANDBOX_SHELL = $ENV{SANDBOX_SHELL}")
+  set(SANDBOX_SHELL "$ENV{SANDBOX_SHELL}")
 else()
   find_program(BUSYBOX busybox)
   if (BUSYBOX)