diff --git a/gapic/schema/wrappers.py b/gapic/schema/wrappers.py
index b1df03310c..55ffb9df07 100644
--- a/gapic/schema/wrappers.py
+++ b/gapic/schema/wrappers.py
@@ -410,8 +410,8 @@ def get_field(self, *field_path: str,
         # binds deeply in the one spot where that might be a problem).
         collisions = collisions or self.meta.address.collisions
 
-        field_path = [
-            name + "_" if name in utils.RESERVED_NAMES else name for name in field_path]
+        field_path = tuple([
+            name + "_" if name in utils.RESERVED_NAMES else name for name in field_path])
 
         # Get the first field in the path.
         cursor = self.fields[field_path[0]]