diff --git a/compiler/semantic/op.go b/compiler/semantic/op.go
index a1bcc22e3d..cc20581d92 100644
--- a/compiler/semantic/op.go
+++ b/compiler/semantic/op.go
@@ -973,7 +973,7 @@ func (a *analyzer) semOpAssignment(p *ast.OpAssignment) (dag.Op, error) {
 		}
 		if _, ok := a.RHS.(*dag.Agg); ok {
 			if _, ok := a.LHS.(*dag.This); !ok {
-				return nil, errors.New("summarize: illegal use of dynamic assignment in aggregation")
+				return nil, errors.New("summarize: aggregate output field must be static")
 			}
 			aggs = append(aggs, a)
 		} else {