diff --git a/core/data_provider.py b/core/data_provider.py
index 40686d4..c6f40cf 100644
--- a/core/data_provider.py
+++ b/core/data_provider.py
@@ -3564,6 +3564,10 @@ class DataProvider:
                 }
                 
                 # Add to cache
+                if symbol not in self.cob_data_cache:
+                    self.cob_data_cache[symbol] = []
+                elif not isinstance(self.cob_data_cache[symbol], (list, deque)):
+                    self.cob_data_cache[symbol] = []
                 self.cob_data_cache[symbol].append(standard_cob_data)
                 if len(self.cob_data_cache[symbol]) > 300:  # Keep 5 minutes
                     self.cob_data_cache[symbol].pop(0)