diff --git a/datasette/filters.py b/datasette/filters.py
index 43cd29f9..4891154a 100644
--- a/datasette/filters.py
+++ b/datasette/filters.py
@@ -191,7 +191,9 @@ class Filters:
     )
     _filters_by_key = {f.key: f for f in _filters}
 
-    def __init__(self, pairs, units={}, ureg=None):
+    def __init__(self, pairs, units=None, ureg=None):
+        if units is None:
+            units = {}
         self.pairs = pairs
         self.units = units
         self.ureg = ureg
diff --git a/datasette/utils/__init__.py b/datasette/utils/__init__.py
index a006f71f..60922957 100644
--- a/datasette/utils/__init__.py
+++ b/datasette/utils/__init__.py
@@ -626,7 +626,11 @@ def module_from_path(path, name):
     return mod
 
 
-async def resolve_table_and_format(table_and_format, table_exists, allowed_formats=[]):
+async def resolve_table_and_format(
+    table_and_format, table_exists, allowed_formats=None
+):
+    if allowed_formats is None:
+        allowed_formats = []
     if "." in table_and_format:
         # Check if a table exists with this exact name
         it_exists = await table_exists(table_and_format)