diff --git a/micropip/_commands/index_urls.py b/micropip/_commands/index_urls.py
index 60cc9b1..aae9cce 100644
--- a/micropip/_commands/index_urls.py
+++ b/micropip/_commands/index_urls.py
@@ -24,4 +24,4 @@ def set_index_urls(urls: list[str] | str) -> None:
     if isinstance(urls, str):
         urls = [urls]
 
-    package_index.INDEX_URLS = urls
+    package_index.INDEX_URLS = urls[:]
diff --git a/micropip/_commands/install.py b/micropip/_commands/install.py
index 155eab0..bc93c1f 100644
--- a/micropip/_commands/install.py
+++ b/micropip/_commands/install.py
@@ -127,7 +127,7 @@ async def install(
     wheel_base = Path(getsitepackages()[0])
 
     if index_urls is None:
-        index_urls = package_index.INDEX_URLS
+        index_urls = package_index.INDEX_URLS[:]
 
     transaction = Transaction(
         ctx=ctx,