diff --git a/pkgs/os-specific/darwin/apple-sdk/private-frameworks-setup-hook.sh b/pkgs/os-specific/darwin/apple-sdk/private-frameworks-setup-hook.sh
index fbd977f8552..a351c39de13 100644
--- a/pkgs/os-specific/darwin/apple-sdk/private-frameworks-setup-hook.sh
+++ b/pkgs/os-specific/darwin/apple-sdk/private-frameworks-setup-hook.sh
@@ -1,6 +1,6 @@
 addPrivateFrameworks() {
     flag="-F/System/Library/PrivateFrameworks"
-    if [[ "$NIX_CFLAGS_COMPILE" != *$flag* ]]; then
+    if [[ "${NIX_CFLAGS_COMPILE-}" != *$flag* ]]; then
         NIX_CFLAGS_COMPILE+=" $flag"
     fi
 }