diff --git a/lib/core/common.py b/lib/core/common.py
index 94961492d..11f12280f 100644
--- a/lib/core/common.py
+++ b/lib/core/common.py
@@ -254,7 +254,7 @@ class BigArray(list):
         index = y / BIGARRAY_CHUNK_LENGTH
         offset = y % BIGARRAY_CHUNK_LENGTH
         chunk = self.chunks[index]
-        if isinstance(chunk, list):
+        if isinstance(chunk, list) and len(chunk) >= offset:
             return chunk[offset]
         else:
             self._checkcache(index)