diff --git a/python/mozperftest/mozperftest/argparser.py b/python/mozperftest/mozperftest/argparser.py
index bceace51852c..009667fe8e4e 100644
--- a/python/mozperftest/mozperftest/argparser.py
+++ b/python/mozperftest/mozperftest/argparser.py
@@ -46,7 +46,7 @@ class Options:
         },
         "--hooks": {
             "type": str,
-            "default": "",
+            "default": None,
             "help": "Script containing hooks. Can be a path or a URL.",
         },
         "--verbose": {"action": "store_true", "default": False, "help": "Verbose mode"},
diff --git a/python/mozperftest/mozperftest/metrics/perfherder.py b/python/mozperftest/mozperftest/metrics/perfherder.py
index 83f6effc8259..66d8ea7689be 100644
--- a/python/mozperftest/mozperftest/metrics/perfherder.py
+++ b/python/mozperftest/mozperftest/metrics/perfherder.py
@@ -45,7 +45,7 @@ class Perfherder(Layer):
                 "refbrow",
             ],
             "help": "Shorthand name of application that is "
-                    "being tested (used in perfherder data).",
+            "being tested (used in perfherder data).",
         },
         "metrics": {
             "nargs": "*",
diff --git a/python/mozperftest/mozperftest/tests/test_browsertime.py b/python/mozperftest/mozperftest/tests/test_browsertime.py
index 2996fff07b64..e4c816aa9289 100644
--- a/python/mozperftest/mozperftest/tests/test_browsertime.py
+++ b/python/mozperftest/mozperftest/tests/test_browsertime.py
@@ -36,10 +36,15 @@ def test_browser(*mocked):
             b(metadata)
     finally:
         shutil.rmtree(mach_cmd._mach_context.state_dir)
-
     assert mach_cmd.run_process.call_count == 1
+
     # XXX more checks
     assert mach_cmd.run_process.call_args[0][-1][-1] == EXAMPLE_TEST
+
+    # Make sure all arguments are of type str
+    for option in mach_cmd.run_process.call_args[0][0]:
+        assert isinstance(option, str)
+
     cmd = " ".join(mach_cmd.run_process.call_args[0][0])
     assert "--firefox.geckodriverPath GECKODRIVER" in cmd
 
diff --git a/python/mozperftest/mozperftest/utils.py b/python/mozperftest/mozperftest/utils.py
index 3ccc8f45523f..f68494488138 100644
--- a/python/mozperftest/mozperftest/utils.py
+++ b/python/mozperftest/mozperftest/utils.py
@@ -132,7 +132,7 @@ def build_test_list(tests, randomized=False):
         test = Path(test)
 
         if test.is_file():
-            res.append(test)
+            res.append(str(test))
         elif test.is_dir():
             for file in test.rglob("perftest_*.js"):
                 res.append(str(file))