diff --git a/spacy/kb.pyx b/spacy/kb.pyx
index f67519260..2b38202f3 100644
--- a/spacy/kb.pyx
+++ b/spacy/kb.pyx
@@ -35,6 +35,13 @@ 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."""
+
+        # Throw an error if the probabilities sum up to more than 1
+        prob_sum = sum(probabilities)
+        if prob_sum > 1:
+            raise ValueError("The sum of prior probabilities for alias '" + alias + "' should not exceed 1, "
+                                                                                    "but found " + str(prob_sum))
+
         cdef hash_t alias_hash = self.strings.add(alias)
 
         # Return if this alias was added before
diff --git a/spacy/sandbox_test_sofie/testing_el.py b/spacy/sandbox_test_sofie/testing_el.py
index 734eddd8d..71fecb7e6 100644
--- a/spacy/sandbox_test_sofie/testing_el.py
+++ b/spacy/sandbox_test_sofie/testing_el.py
@@ -42,6 +42,12 @@ def create_kb():
 
     print("kb size", len(mykb), mykb.get_size_entities(), mykb.get_size_aliases())
 
+    alias2 = "johny"
+    print(" adding alias2", alias2)
+    mykb.add_alias(alias=alias2, entities=["Q0", "Q42"], probabilities=[0.3, 1.1])
+
+    print("kb size", len(mykb), mykb.get_size_entities(), mykb.get_size_aliases())
+
     print("candidates for", alias)
     candidates = mykb.get_candidates(alias)
     print(" ", candidates)
diff --git a/spacy/tests/pipeline/test_el.py b/spacy/tests/pipeline/test_el.py
index ed88076ce..f9533ef82 100644
--- a/spacy/tests/pipeline/test_el.py
+++ b/spacy/tests/pipeline/test_el.py
@@ -1,14 +1,16 @@
+# coding: utf-8
 import pytest
 
 from spacy.kb import KnowledgeBase
 
 
 def test_kb_valid_entities():
+    """Test the valid construction of a KB with 3 entities and one alias"""
     mykb = KnowledgeBase()
 
     # adding entities
-    mykb.add_entity(entity_id="Q1", prob=0.5)
-    mykb.add_entity(entity_id="Q2", prob=0.5)
+    mykb.add_entity(entity_id="Q1", prob=0.9)
+    mykb.add_entity(entity_id="Q2", prob=0.2)
     mykb.add_entity(entity_id="Q3", prob=0.5)
 
     # adding aliases
@@ -16,14 +18,29 @@ def test_kb_valid_entities():
 
 
 def test_kb_invalid_entities():
+    """Test the invalid construction of a KB with an alias linked to a non-existing entity"""
     mykb = KnowledgeBase()
 
     # adding entities
-    mykb.add_entity(entity_id="Q1", prob=0.5)
-    mykb.add_entity(entity_id="Q2", prob=0.5)
+    mykb.add_entity(entity_id="Q1", prob=0.9)
+    mykb.add_entity(entity_id="Q2", prob=0.2)
     mykb.add_entity(entity_id="Q3", prob=0.5)
 
     # adding aliases - should fail because one of the given IDs is not valid
     with pytest.raises(ValueError):
         mykb.add_alias(alias="douglassss", entities=["Q2", "Q342"], probabilities=[0.8, 0.2])
 
+
+def test_kb_invalid_probabilities():
+    """Test the invalid construction of a KB with wrong prior probabilities"""
+    mykb = KnowledgeBase()
+
+    # adding entities
+    mykb.add_entity(entity_id="Q1", prob=0.9)
+    mykb.add_entity(entity_id="Q2", prob=0.2)
+    mykb.add_entity(entity_id="Q3", prob=0.5)
+
+    # adding aliases - should fail because the sum of the probabilities exceeds 1
+    with pytest.raises(ValueError):
+        mykb.add_alias(alias="douglassss", entities=["Q2", "Q3"], probabilities=[0.8, 0.4])
+