diff --git a/r2/r2/models/traffic.py b/r2/r2/models/traffic.py
index 4fd94646c..c8ff2f7cc 100644
--- a/r2/r2/models/traffic.py
+++ b/r2/r2/models/traffic.py
@@ -39,8 +39,14 @@ from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import scoped_session, sessionmaker
 from sqlalchemy.schema import Column
 from sqlalchemy.sql.expression import desc, distinct
-from sqlalchemy.sql.functions import sum
-from sqlalchemy.types import DateTime, Integer, String, BigInteger
+from sqlalchemy.sql.functions import sum as sa_sum
+from sqlalchemy.types import (
+    BigInteger,
+    DateTime,
+    Integer,
+    String,
+    TypeDecorator,
+)
 
 from r2.lib.memoize import memoize
 from r2.lib.utils import timedelta_by_name, tup
@@ -252,6 +258,32 @@ def top_last_month(cls, key):
             for r in q.all()]
 
 
+class CoerceToLong(TypeDecorator):
+    # source:
+    # https://groups.google.com/forum/?fromgroups=#!topic/sqlalchemy/3fipkThttQA
+
+    impl = BigInteger
+
+    def process_result_value(self, value, dialect):
+        if value is not None:
+            value = long(value)
+        return value
+
+
+def sum(column):
+    """Wrapper around sqlalchemy.sql.functions.sum to handle BigInteger.
+
+    sqlalchemy returns a Decimal for sum over BigInteger values. Detect the
+    column type and coerce to long if it's a BigInteger.
+
+    """
+
+    if isinstance(column.property.columns[0].type, BigInteger):
+        return sa_sum(column, type_=CoerceToLong)
+    else:
+        return sa_sum(column)
+
+
 def totals(cls, interval):
     """Aggregate sitewide totals for self-serve promotion traffic.