diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
index c1be2f4b1..8318013c7 100644
--- a/include/nlohmann/json.hpp
+++ b/include/nlohmann/json.hpp
@@ -3653,6 +3653,13 @@ class basic_json
             // fill up array with null values if given idx is outside range
             if (idx >= m_value.array->size())
             {
+#if JSON_DIAGNOSTICS
+                // remember array size before resizing
+                const auto previous_size = m_value.array->size();
+#endif
+
+                m_value.array->resize(idx + 1);
+
 #if JSON_DIAGNOSTICS
                 // set parent for values added above
                 for (auto i = previous_size; i <= idx; ++i)