diff --git a/sql/expression/function/aggregation/unary_agg_buffers.go b/sql/expression/function/aggregation/unary_agg_buffers.go
index 1e7daa6571..c9a3aa47c9 100644
--- a/sql/expression/function/aggregation/unary_agg_buffers.go
+++ b/sql/expression/function/aggregation/unary_agg_buffers.go
@@ -90,18 +90,41 @@ func (m *sumBuffer) PerformSum(ctx *sql.Context, v interface{}) {
 			v = val
 		}
 	}
-
 	switch n := v.(type) {
+	case float64:
+		if m.isnil {
+			m.sum = float64(0)
+			m.isnil = false
+		}
+		switch sum := m.sum.(type) {
+		case float64:
+		case decimal.Decimal:
+			m.sum, _ = sum.Float64()
+		default:
+			var err error
+			m.sum, _, err = types.Float64.Convert(ctx, sum)
+			if err != nil {
+				m.sum = float64(0)
+			}
+		}
+		m.sum = m.sum.(float64) + n
 	case decimal.Decimal:
 		if m.isnil {
 			m.sum = decimal.NewFromInt(0)
 			m.isnil = false
 		}
-		if sum, ok := m.sum.(decimal.Decimal); ok {
-			m.sum = sum.Add(n)
-		} else {
-			m.sum = decimal.NewFromFloat(m.sum.(float64)).Add(n)
+		switch sum := m.sum.(type) {
+		case decimal.Decimal:
+		case float64:
+			m.sum = decimal.NewFromFloat(sum)
+		default:
+			var err error
+			m.sum, _, err = types.InternalDecimalType.Convert(ctx, sum)
+			if err != nil {
+				m.sum = decimal.NewFromInt(0)
+			}
 		}
+		m.sum = m.sum.(decimal.Decimal).Add(n)
 	default:
 		val, _, err := types.Float64.Convert(ctx, n)
 		if err != nil {
@@ -111,11 +134,17 @@ func (m *sumBuffer) PerformSum(ctx *sql.Context, v interface{}) {
 			m.sum = float64(0)
 			m.isnil = false
 		}
-		sum, _, err := types.Float64.Convert(ctx, m.sum)
-		if err != nil {
-			sum = float64(0)
+		switch sum := m.sum.(type) {
+		case float64:
+		case decimal.Decimal:
+			m.sum, _ = sum.Float64()
+		default:
+			sum, _, err = types.Float64.Convert(ctx, sum)
+			if err != nil {
+				sum = float64(0)
+			}
 		}
-		m.sum = sum.(float64) + val.(float64)
+		m.sum = m.sum.(float64) + val.(float64)
 	}
 }