diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -275,7 +275,8 @@
 def run_tests(test_list, src_dir, build_dir, exeext, tmpdir, jobs=1, enable_coverage=False, args=[]):
     # Warn if bitcoind is already running (unix only)
     try:
-        if subprocess.check_output(["pidof", "bitcoind"]) is not None:
+        pidofOutput = subprocess.check_output(["pidof", "bitcoind"])
+        if pidofOutput is not None and pidofOutput != 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):