diff --git a/runtime/expr/coerce/coerce.go b/runtime/expr/coerce/coerce.go
index 6b97def0a0..1fc3a0634e 100644
--- a/runtime/expr/coerce/coerce.go
+++ b/runtime/expr/coerce/coerce.go
@@ -48,6 +48,7 @@ func (c *Pair) Equal() bool {
 }
 
 func (c *Pair) Coerce(a, b zed.Value) (int, error) {
+	a, b = a.Under(), b.Under()
 	c.A = a.Bytes()
 	c.B = b.Bytes()
 	aid := a.Type().ID()
diff --git a/runtime/expr/expr_test.go b/runtime/expr/expr_test.go
index 86a1ad6a00..08916d6033 100644
--- a/runtime/expr/expr_test.go
+++ b/runtime/expr/expr_test.go
@@ -355,6 +355,14 @@ func TestArithmetic(t *testing.T) {
 	testSuccessful(t, "f / 1.25", record, zed.NewFloat64(2.0))
 	testSuccessful(t, "5.0 / f", record, zed.NewFloat64(2.0))
 
+	// Union values are dereferenced when doing arithmetic on them.
+	val := "10((int64,string))"
+	testSuccessful(t, "this + 1", val, zed.NewInt64(11))
+	testSuccessful(t, "this - 1", val, zed.NewInt64(9))
+	testSuccessful(t, "this * 2", val, zed.NewInt64(20))
+	testSuccessful(t, "this / 2", val, zed.NewInt64(5))
+	testSuccessful(t, "this % 3", val, zed.NewInt64(1))
+
 	// Difference of two times is a duration
 	testSuccessful(t, "a - b", "{a:2022-09-22T00:00:01Z,b:2022-09-22T00:00:00Z}",
 		zed.NewDuration(nano.Second))