diff --git a/large_image/tilesource/base.py b/large_image/tilesource/base.py
index e5a923d5b..dde193188 100644
--- a/large_image/tilesource/base.py
+++ b/large_image/tilesource/base.py
@@ -1322,7 +1322,7 @@ def _getFrameValueInformation(self, frames: List[Dict]):
                 if 'Value' in key:
                     if key not in refvalues:
                         refvalues[key] = {}
-                    value_index = frame.get(key.replace('Value', 'Index'))
+                    value_index: str = frame.get(key.replace('Value', 'Index'))
                     if value_index not in refvalues[key]:
                         refvalues[key][value_index] = [value]
                     else: