diff --git a/src/pgmon.py b/src/pgmon.py
index 58a4a9e..d184a67 100755
--- a/src/pgmon.py
+++ b/src/pgmon.py
@@ -426,7 +426,7 @@ def run_query_no_retry(pool, return_type, query, args):
                         return "[]"
                     return json.dumps([list(r.values())[0] for r in res], default=json_encode_special)
                 elif return_type == "set":
-                    return json.dumps(res)
+                    return json.dumps(res, default=json_encode_special)
         except:
             dbname = pool.name
             if dbname in unhappy_cooldown:
diff --git a/src/test_pgmon.py b/src/test_pgmon.py
index 7e88c8d..1a86492 100644
--- a/src/test_pgmon.py
+++ b/src/test_pgmon.py
@@ -6,6 +6,7 @@ import tempfile
 import logging
 
 from decimal import Decimal
+import json
 
 import pgmon
 
@@ -805,3 +806,6 @@ metrics:
         self.assertRaises(
             TypeError, pgmon.json_encode_special, object
         )
+
+        # Make sure we can actually serialize a Decimal
+        self.assertEqual(json.dumps(Decimal('2.5'), default=pgmon.json_encode_special), '2.5')