diff --git a/neo/core/baseneo.py b/neo/core/baseneo.py
index 3bd49e5ca..cc031b03e 100644
--- a/neo/core/baseneo.py
+++ b/neo/core/baseneo.py
@@ -45,7 +45,9 @@ def _check_annotations(value):
         if not issubclass(value.dtype.type, ALLOWED_ANNOTATION_TYPES):
             raise ValueError(f"Invalid annotation. NumPy arrays with dtype {value.dtype.type}" f"are not allowed")
     elif isinstance(value, dict):
-        for element in value.values():
+        for key, element in value.items():
+            if not isinstance(key, str):
+                raise TypeError(f"Annotations keys must be strings not of type {type(key)}")
             _check_annotations(element)
     elif isinstance(value, (list, tuple)):
         for element in value: