diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 9b4a5d203..4156e2272 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -364,9 +364,10 @@ def main():
 def run_tests(*, test_list, src_dir, build_dir, tmpdir, jobs=1, enable_coverage=False, args=None, combined_logs_len=0, failfast=False, runs_ci, use_term_control):
     args = args or []
 
-    # Warn if bitcoind is already running (unix only)
+    # Warn if bitcoind is already running
+    # pidof might fail or return an empty string if bitcoind is not running
     try:
-        if subprocess.check_output(["pidof", "bitcoind"]) is not None:
+        if subprocess.check_output(["pidof", "bitcoind"]) not in [b'']:
             print("%sWARNING!%s There is already a bitcoind process running on this system. Tests may fail unexpectedly due to resource contention!" % (BOLD[1], BOLD[0]))
     except (OSError, subprocess.SubprocessError):
         pass