Fix mypy error.

spacy/errors.py

@@ -952,6 +952,7 @@ class Errors(metaclass=ErrorsWithCodes):
              "with `displacy.serve(doc, port=port)`")
     E1050 = ("Port {port} is already in use. Please specify an available port with `displacy.serve(doc, port=port)` "
              "or use `auto_switch_port=True` to pick an available port automatically.")
+    E1051 = ("Expected `entity_id` to be of type {should_type}, but is of type {is_type}.")
 
     # v4 error strings
     E4000 = ("Expected a Doc as input, but got: '{type}'")

spacy/kb/candidate.py

@@ -1,6 +1,8 @@
 import abc
 from typing import List, Union, Callable
 
+from ..errors import Errors
+
 
 class Candidate(abc.ABC):
     """A `Candidate` object refers to a textual mention that may or may not be resolved
@@ -97,6 +99,10 @@ class InMemoryCandidate(Candidate):
         )
         self._hash_to_str = hash_to_str
         self._entity_freq = entity_freq
+        if not isinstance(self._entity_id, int):
+            raise ValueError(
+                Errors.E1051.format(should_type="int", is_type=str(type(entity_id)))
+            )
         self._entity_id_str = self._hash_to_str(self._entity_id)
 
     @property