diff --git a/toolchain/defs.bzl b/toolchain/defs.bzl
index b761b60..0e4b4e8 100644
--- a/toolchain/defs.bzl
+++ b/toolchain/defs.bzl
@@ -175,7 +175,7 @@ ZIG_TOOL_PATH = "tools/{zig_tool}"
 ZIG_TOOL_WRAPPER = """#!/bin/bash
 set -e
 
-if [[ -z "$TMPDIR" ]]; then
+if [[ -n "$TMPDIR" ]]; then
   _cache_prefix=$TMPDIR
 else
   _cache_prefix="$HOME/.cache"