diff --git a/api/chalicelib/core/sourcemaps.py b/api/chalicelib/core/sourcemaps.py
index 4ec973886..12c2bd96a 100644
--- a/api/chalicelib/core/sourcemaps.py
+++ b/api/chalicelib/core/sourcemaps.py
@@ -89,7 +89,7 @@ def get_traces_group(project_id, payload):
         file_exists_in_server = False
         file_url = u["absPath"]
         key = __get_key(project_id, file_url)  # use filename instead?
-        if file_url and len(file_url) > 0 and not file_url[:file_url.index("?")].endswith(".js"):
+        if file_url and len(file_url) > 0 and not file_url[:file_url.find("?")].endswith(".js"):
             print(f"{u['absPath']} sourcemap is not a JS file")
             payloads[key] = None
             continue