diff --git a/runtime/vam/expr/function/upcast.go b/runtime/vam/expr/function/upcast.go
index 98bcc5af4..fd7c1c1ca 100644
--- a/runtime/vam/expr/function/upcast.go
+++ b/runtime/vam/expr/function/upcast.go
@@ -214,14 +214,30 @@ func (u *Upcast) toUnion(vec vector.Any, to *super.TypeUnion) vector.Any {
 	if unionVec, ok := vec.(*vector.Union); ok {
 		types := map[super.Type]struct{}{}
 		values := make([]vector.Any, 0, len(to.Types))
+		var needMerge bool
 		for _, vec := range unionVec.Values() {
 			vec = u.toUnionValue(vec, to)
 			if vec == nil {
 				return nil
 			}
-			types[vec.Type()] = struct{}{}
+			typ := vec.Type()
+			if _, ok := types[typ]; !ok {
+				needMerge = true
+			}
+			types[typ] = struct{}{}
 			values = append(values, vec)
 		}
+		tags := unionVec.Tags()
+		if needMerge {
+			mergedVec := vbuild.MergeSameTypesInDynamic(vector.NewDynamic(tags, values))
+			if d, ok := mergedVec.(*vector.Dynamic); ok {
+				tags, values = d.Tags, d.Values
+			} else {
+				tags = make([]uint32, mergedVec.Len())
+				values = values[:1]
+				values[0] = mergedVec
+			}
+		}
 		for _, typ := range to.Types {
 			if _, ok := types[typ]; !ok {
 				values = append(values, vector.NewEmpty(typ))
@@ -229,7 +245,7 @@ func (u *Upcast) toUnion(vec vector.Any, to *super.TypeUnion) vector.Any {
 		}
 		//XXX We should copy RLE instead of making tags when we can.
 		// This will be addressed in a subsequent PR.
-		return vector.NewUnion(to, unionVec.Tags(), values)
+		return vector.NewUnion(to, tags, values)
 	}
 	values := u.toUnionValue(vec, to)
 	if values == nil {
diff --git a/runtime/ztests/expr/function/defuse.yaml b/runtime/ztests/expr/function/defuse.yaml
index 7d613d77d..9bcb4162b 100644
--- a/runtime/ztests/expr/function/defuse.yaml
+++ b/runtime/ztests/expr/function/defuse.yaml
@@ -211,6 +211,16 @@ output: *input
 
 spq: fuse | defuse(this)
 
+input: &input |
+  1::(int64|[int64]|[string])
+  [2]::(int64|[int64]|[string])
+
+output: *input
+
+---
+
+spq: fuse | defuse(this)
+
 input: &input |
   [{a:1}]
   [{a:3},{b:3}]