diff --git a/newsfragments/3739.bugfix b/newsfragments/3739.bugfix
new file mode 100644
index 000000000..875941cf8
--- /dev/null
+++ b/newsfragments/3739.bugfix
@@ -0,0 +1 @@
+Fixed annoying UnicodeWarning message on Python 2 when running CLI tools.
\ No newline at end of file
diff --git a/src/allmydata/scripts/runner.py b/src/allmydata/scripts/runner.py
index 43a3b8807..7f7f88bf6 100644
--- a/src/allmydata/scripts/runner.py
+++ b/src/allmydata/scripts/runner.py
@@ -204,7 +204,8 @@ def _setup_coverage(reactor):
     """
     # can we put this _setup_coverage call after we hit
     # argument-parsing?
-    if '--coverage' not in sys.argv:
+    # ensure_str() only necessary on Python 2.
+    if six.ensure_str('--coverage') not in sys.argv:
         return
     sys.argv.remove('--coverage')