diff --git a/spacy/kb.pxd b/spacy/kb.pxd
index d96502f41..9f0a5e68d 100644
--- a/spacy/kb.pxd
+++ b/spacy/kb.pxd
@@ -98,18 +98,10 @@ cdef class KnowledgeBase:
         self._entry_index[entity_key] = entity_index
         return entity_index
 
-    cdef inline int64_t c_add_aliases(self, hash_t alias_key, entities, probabilities):
+    cdef inline int64_t c_add_aliases(self, hash_t alias_key, vector[int64_t] entry_indices, vector[float] probs):
         """Connect a mention to a list of potential entities with their prior probabilities ."""
         cdef int64_t alias_index = self._aliases_table.size()
 
-        cdef vector[int64_t] entry_indices
-        cdef vector[float] probs
-
-        for entity, prob in zip(entities, probs):
-            entry_index = self._entry_index[hash_string(entity)]
-            entry_indices.push_back(entry_index)
-            probs.push_back(prob)
-
         self._aliases_table.push_back(
             _AliasC(
                 entry_indices=entry_indices,
diff --git a/spacy/kb.pyx b/spacy/kb.pyx
index b4369d59b..854feb069 100644
--- a/spacy/kb.pyx
+++ b/spacy/kb.pyx
@@ -35,13 +35,23 @@ cdef class KnowledgeBase:
     def add_alias(self, unicode alias, entities, probabilities):
         """For a given alias, add its potential entities and prior probabilies to the KB."""
         cdef hash_t alias_hash = hash_string(alias)
+        cdef hash_t entity_hash
+
+        cdef vector[int64_t] entry_indices
+        cdef vector[float] probs
+
+        for entity, prob in zip(entities, probabilities):
+            entity_hash = hash_string(entity)
+            entry_index = <int64_t>self._entry_index.get(entity_hash)
+            entry_indices.push_back(int(entry_index))
+            probs.push_back(float(prob))
 
         # TODO: check that alias hadn't been defined before
         # TODO: check that entity is already in this KB (entity_index is OK)
         # TODO: check sum(probabilities) <= 1
         # TODO: check len(entities) == len(probabilities)
 
-        self.c_add_aliases(alias_key=alias_hash, entities=entities, probabilities=probabilities)
+        self.c_add_aliases(alias_key=alias_hash, entry_indices=entry_indices, probs=probs)
 
     def get_candidates(self, unicode alias):
         cdef hash_t alias_hash = hash_string(alias)