diff --git a/pkgs/development/tools/build-managers/cmake/001-search-path.diff b/pkgs/development/tools/build-managers/cmake/001-search-path.diff
index 607668b131556..04ab0847a7074 100644
--- a/pkgs/development/tools/build-managers/cmake/001-search-path.diff
+++ b/pkgs/development/tools/build-managers/cmake/001-search-path.diff
@@ -12,7 +12,7 @@ index b9381c3d7d..5e944640b5 100644
    # CMake install location
    "${_CMAKE_INSTALL_DIR}"
    )
-@@ -47,48 +44,48 @@ endif()
+@@ -47,48 +44,49 @@ endif()
  
  # Non "standard" but common install prefixes
  list(APPEND CMAKE_SYSTEM_PREFIX_PATH
@@ -22,9 +22,10 @@ index b9381c3d7d..5e944640b5 100644
    )
  
  # List common include file locations not under the common prefixes.
-+if(IS_DIRECTORY $ENV{NIX_CC}
-+  AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc
-+  AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc-dev)
++if(DEFINED ENV{NIX_CC}
++  AND IS_DIRECTORY "$ENV{NIX_CC}"
++  AND EXISTS "$ENV{NIX_CC}/nix-support/orig-libc"
++  AND EXISTS "$ENV{NIX_CC}/nix-support/orig-libc-dev")
 +  file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc" _nix_cmake_libc)
 +  file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc-dev" _nix_cmake_libc_dev)
 +else()