diff --git a/CHANGES.txt b/CHANGES.txt
index 201ff167d0d..3ee354b3a8b 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -11,6 +11,9 @@ Avro 1.3.3 (Unreleased)
 
     AVRO-560. Python impl should include system errors in every protocol (hammer)
 
+    AVRO-559. Handle read_union error where the list index of the union branch 
+    to follow exceeds the size of the union schema (hammer)
+
   BUG FIXES
 
     AVRO-496. python sample_http_client.py is broken (Jeff Hodges via hammer)
diff --git a/lang/py/src/avro/io.py b/lang/py/src/avro/io.py
index ff2ba24faf9..55e15639b17 100644
--- a/lang/py/src/avro/io.py
+++ b/lang/py/src/avro/io.py
@@ -606,6 +606,10 @@ def read_union(self, writers_schema, readers_schema, decoder):
     """
     # schema resolution
     index_of_schema = int(decoder.read_long())
+    if index_of_schema >= len(writers_schema.schemas):
+      fail_msg = "Can't access branch index %d for union with %d branches"\
+                 % (index_of_schema, writers_schema.schemas)
+      raise SchemaResolutionException(fail_msg, writers_schema, readers_schema)
     selected_writers_schema = writers_schema.schemas[index_of_schema]
     
     # read data