diff --git a/promql/parser/printer.go b/promql/parser/printer.go
index e1f8775301..9dae10a70e 100644
--- a/promql/parser/printer.go
+++ b/promql/parser/printer.go
@@ -157,7 +157,15 @@ func (node *DurationExpr) String() string {
 		expr = fmt.Sprintf("max(%s, %s)", node.LHS, node.RHS)
 	case node.LHS == nil:
 		// This is a unary duration expression.
-		expr = fmt.Sprintf("%s%s", node.Op, node.RHS)
+		switch node.Op {
+		case SUB:
+			expr = fmt.Sprintf("%s%s", node.Op, node.RHS)
+		case ADD:
+			expr = node.RHS.String()
+		default:
+			// This should never happen.
+			panic(fmt.Sprintf("unexpected unary duration expression: %s", node.Op))
+		}
 	default:
 		expr = fmt.Sprintf("%s %s %s", node.LHS, node.Op, node.RHS)
 	}
diff --git a/promql/parser/printer_test.go b/promql/parser/printer_test.go
index 9b4d3d8371..d9a091d78b 100644
--- a/promql/parser/printer_test.go
+++ b/promql/parser/printer_test.go
@@ -199,6 +199,10 @@ func TestExprString(t *testing.T) {
 			in:  "foo offset +(5*2)",
 			out: "foo offset (5 * 2)",
 		},
+		{
+			in:  "foo offset +min(10s, 20s)",
+			out: "foo offset min(10s, 20s)",
+		},
 		{
 			in: "foo offset -min(10s, 20s)",
 		},