diff --git a/cloudkitty/storage/v2/opensearch/client.py b/cloudkitty/storage/v2/opensearch/client.py
index 0b929552..d5757c9b 100644
--- a/cloudkitty/storage/v2/opensearch/client.py
+++ b/cloudkitty/storage/v2/opensearch/client.py
@@ -98,14 +98,15 @@ class OpenSearchClient(object):
         sources = []
         for elem in groupby:
             if elem == 'type':
-                sources.append({'type': {'terms': {'field': 'type'}}})
+                sources.append({'type': {'terms': {'field': 'type.keyword'}}})
             elif elem == 'time':
                 # Not doing a date_histogram aggregation because we don't know
                 # the period
                 sources.append({'begin': {'terms': {'field': 'start'}}})
                 sources.append({'end': {'terms': {'field': 'end'}}})
             else:
-                sources.append({elem: {'terms': {'field': 'groupby.' + elem}}})
+                field = 'groupby.' + elem + '.keyword'
+                sources.append({elem: {'terms': {'field': field}}})
 
         return {"sources": sources}
 
diff --git a/cloudkitty/tests/storage/v2/opensearch/test_client.py b/cloudkitty/tests/storage/v2/opensearch/test_client.py
index 2fba72dc..b33515c6 100644
--- a/cloudkitty/tests/storage/v2/opensearch/test_client.py
+++ b/cloudkitty/tests/storage/v2/opensearch/test_client.py
@@ -86,9 +86,9 @@ class TestOpenSearchClient(unittest.TestCase):
         self.assertEqual(
             self.client._build_composite(['one', 'type', 'two']),
             {'sources': [
-                {'one': {'terms': {'field': 'groupby.one'}}},
-                {'type': {'terms': {'field': 'type'}}},
-                {'two': {'terms': {'field': 'groupby.two'}}},
+                {'one': {'terms': {'field': 'groupby.one.keyword'}}},
+                {'type': {'terms': {'field': 'type.keyword'}}},
+                {'two': {'terms': {'field': 'groupby.two.keyword'}}},
             ]},
         )
 
diff --git a/releasenotes/notes/fix-opensearch-report-344508dd4e3d0ccc.yaml b/releasenotes/notes/fix-opensearch-report-344508dd4e3d0ccc.yaml
new file mode 100644
index 00000000..7f1963fc
--- /dev/null
+++ b/releasenotes/notes/fix-opensearch-report-344508dd4e3d0ccc.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix some API report requests that were returning HTTP 500 errors when using
+    the ``opensearch`` storage backend. This fixes failures to load the Horizon
+    ``Rating`` panel.