diff --git a/tools/toolhelper.py b/tools/toolhelper.py
index 806205f..cc1a9cb 100644
--- a/tools/toolhelper.py
+++ b/tools/toolhelper.py
@@ -3,21 +3,24 @@
 # BSD-style license that can be found in the LICENSE file.
 
 import glob
-import gradle
-import jdk
 import subprocess
 from threading import Timer
+
+import gradle
+import jdk
 import utils
 
+
 def run(tool, args, build=None, debug=True,
         profile=False, track_memory_file=None, extra_args=None,
         stderr=None, stdout=None, return_stdout=False, timeout=0, quiet=False,
         cmd_prefix=[]):
+  cmd = []
+  cmd.extend(cmd_prefix)
   if build is None:
     build, args = extract_build_from_args(args)
   if build:
     gradle.RunGradle(['r8lib' if tool.startswith('r8lib') else 'r8'])
-  cmd = cmd_prefix
   if track_memory_file:
     cmd.extend(['tools/track_memory.sh', track_memory_file])
   cmd.append(jdk.GetJavaExecutable())