diff --git a/patch.go b/patch.go
index 16bcbf6..0c8f7b0 100644
--- a/patch.go
+++ b/patch.go
@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"reflect"
+	"strconv"
 	"strings"
 )
 
@@ -156,6 +157,11 @@ func (v *Value) patchAdd(i int, op patchOperation) error {
 				comp.Members[s.idx].Name.Value = String(s.name)
 			}
 		case *Array:
+			// Per RFC 6902, section 4.1, the index must not be greater
+			// than the number of elements in the array.
+			if n, err := strconv.ParseUint(s.name, 10, 0); s.name != "-" && (err != nil || n != uint64(s.idx)) {
+				return fmt.Errorf("hujson: patch operation %d: array index out of range: %s", i, s.name)
+			}
 			insertAt(comp, s.idx, op.value)
 		}
 	}
diff --git a/patch_test.go b/patch_test.go
index 4808d6b..56d1580 100644
--- a/patch_test.go
+++ b/patch_test.go
@@ -179,6 +179,19 @@ var testdataPatch = []struct {
 	in:      `{"fizz":["buzz","wuzz"],"fizzy":"wizzy"}`,
 	patch:   `[{ "op": "test", "path": "/noexist", "value": null }]`,
 	wantErr: errors.New(`hujson: patch operation 0: value not found`),
+}, {
+	in:    `[1,2]`,
+	patch: `[{ "op": "add", "path": "/2", "value": 3 }]`,
+	want:  `[1,2,3]`,
+}, {
+	in:    `[1,2]`,
+	patch: `[{ "op": "add", "path": "/-", "value": 3 }]`,
+	want:  `[1,2,3]`,
+}, {
+	in:      `[1,2]`,
+	patch:   `[{ "op": "add", "path": "/5", "value": 3 }]`,
+	wantErr: errors.New(`hujson: patch operation 0: array index out of range: 5`),
+	want:    `[1,2]`,
 }, {
 	in:      `{}`,
 	patch:   `[{`,