diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6276029c..526c10e5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,12 @@
 ArduinoJson: change log
 =======================
 
+HEAD
+----
+
+* Fixed `invalid application of 'sizeof' to incomplete type '__FlashStringHelper'` (issue #783)
+* Fixed `char[]` not duplicated when passed to `JsonVariant::operator[]`
+
 v6.2.1-beta
 -----------
 
diff --git a/src/ArduinoJson/JsonObjectSubscript.hpp b/src/ArduinoJson/JsonObjectSubscript.hpp
index e9d8c0dc..fba6e6d4 100644
--- a/src/ArduinoJson/JsonObjectSubscript.hpp
+++ b/src/ArduinoJson/JsonObjectSubscript.hpp
@@ -111,17 +111,17 @@ inline typename enable_if<IsString<TString>::value,
 
 template <typename TImpl>
 template <typename TString>
-inline typename enable_if<IsString<const TString *>::value,
-                          JsonObjectSubscript<const TString *> >::type
-    JsonVariantSubscripts<TImpl>::operator[](const TString *key) {
+inline typename enable_if<IsString<TString *>::value,
+                          JsonObjectSubscript<TString *> >::type
+    JsonVariantSubscripts<TImpl>::operator[](TString *key) {
   return impl()->template as<JsonObject>()[key];
 }
 
 template <typename TImpl>
 template <typename TString>
 inline typename enable_if<IsString<TString *>::value,
-                          const JsonObjectSubscript<const TString *> >::type
-    JsonVariantSubscripts<TImpl>::operator[](const TString *key) const {
+                          const JsonObjectSubscript<TString *> >::type
+    JsonVariantSubscripts<TImpl>::operator[](TString *key) const {
   return impl()->template as<JsonObject>()[key];
 }
 
diff --git a/src/ArduinoJson/JsonVariantSubscripts.hpp b/src/ArduinoJson/JsonVariantSubscripts.hpp
index d2aa3aa5..3b5a5eb1 100644
--- a/src/ArduinoJson/JsonVariantSubscripts.hpp
+++ b/src/ArduinoJson/JsonVariantSubscripts.hpp
@@ -57,17 +57,16 @@ class JsonVariantSubscripts {
   // JsonObjectSubscript operator[](TKey);
   // TKey = const char*, const char[N], const FlashStringHelper*
   template <typename TString>
-  FORCE_INLINE typename enable_if<IsString<const TString *>::value,
-                                  JsonObjectSubscript<const TString *> >::type
-  operator[](const TString *key);
+  FORCE_INLINE typename enable_if<IsString<TString *>::value,
+                                  JsonObjectSubscript<TString *> >::type
+  operator[](TString *key);
   //
   // JsonObjectSubscript operator[](TKey);
   // TKey = const char*, const char[N], const FlashStringHelper*
   template <typename TString>
-  FORCE_INLINE
-      typename enable_if<IsString<TString *>::value,
-                         const JsonObjectSubscript<const TString *> >::type
-      operator[](const TString *key) const;
+  FORCE_INLINE typename enable_if<IsString<TString *>::value,
+                                  const JsonObjectSubscript<TString *> >::type
+  operator[](TString *key) const;
 
  private:
   const TImpl *impl() const {
diff --git a/test/JsonVariant/subscript.cpp b/test/JsonVariant/subscript.cpp
index c2192466..24227252 100644
--- a/test/JsonVariant/subscript.cpp
+++ b/test/JsonVariant/subscript.cpp
@@ -6,79 +6,89 @@
 #include <catch.hpp>
 
 TEST_CASE("JsonVariant::operator[]") {
-  SECTION("Array") {
-    DynamicJsonDocument doc;
-    JsonArray array = doc.to<JsonArray>();
-    array.add("element at index 0");
-    array.add("element at index 1");
-
-    JsonVariant var = array;
-
-    REQUIRE(2 == var.size());
-    REQUIRE(std::string("element at index 0") == var[0]);
-    REQUIRE(std::string("element at index 1") == var[1]);
-    REQUIRE(std::string("element at index 0") ==
-            var[static_cast<unsigned char>(0)]);  // issue #381
-    REQUIRE(var[666].isNull());
-    REQUIRE(var[3].isNull());
-    REQUIRE(var["0"].isNull());
-  }
-
-  SECTION("Object") {
-    DynamicJsonDocument doc;
-    JsonObject object = doc.to<JsonObject>();
-    object["a"] = "element at key \"a\"";
-    object["b"] = "element at key \"b\"";
-
-    JsonVariant var = object;
-
-    REQUIRE(2 == var.size());
-    REQUIRE(std::string("element at key \"a\"") == var["a"]);
-    REQUIRE(std::string("element at key \"b\"") == var["b"]);
-    REQUIRE(var["c"].isNull());
-    REQUIRE(var[0].isNull());
-  }
-
-  SECTION("Undefined") {
+  SECTION("The JsonVariant is undefined") {
     JsonVariant var = JsonVariant();
     REQUIRE(0 == var.size());
     REQUIRE(var["0"].isNull());
     REQUIRE(var[0].isNull());
   }
 
-  SECTION("String") {
+  SECTION("The JsonVariant is a string") {
     JsonVariant var = "hello world";
     REQUIRE(0 == var.size());
     REQUIRE(var["0"].isNull());
     REQUIRE(var[0].isNull());
   }
 
-  SECTION("ObjectSetValue") {
+  SECTION("The JsonVariant is a JsonArray") {
     DynamicJsonDocument doc;
-    JsonObject obj = doc.to<JsonObject>();
-    JsonVariant var = obj;
-    var["hello"] = "world";
-    REQUIRE(1 == var.size());
-    REQUIRE(std::string("world") == var["hello"]);
+    JsonArray array = doc.to<JsonArray>();
+    JsonVariant var = array;
+
+    SECTION("get value") {
+      array.add("element at index 0");
+      array.add("element at index 1");
+
+      REQUIRE(2 == var.size());
+      REQUIRE(std::string("element at index 0") == var[0]);
+      REQUIRE(std::string("element at index 1") == var[1]);
+      REQUIRE(std::string("element at index 0") ==
+              var[static_cast<unsigned char>(0)]);  // issue #381
+      REQUIRE(var[666].isNull());
+      REQUIRE(var[3].isNull());
+      REQUIRE(var["0"].isNull());
+    }
+
+    SECTION("set value") {
+      array.add("hello");
+
+      var[0] = "world";
+
+      REQUIRE(1 == var.size());
+      REQUIRE(std::string("world") == var[0]);
+    }
+
+    SECTION("set value in a nested object") {
+      array.createNestedObject();
+
+      var[0]["hello"] = "world";
+
+      REQUIRE(1 == var.size());
+      REQUIRE(1 == var[0].size());
+      REQUIRE(std::string("world") == var[0]["hello"]);
+    }
   }
 
-  SECTION("ArraySetValue") {
+  SECTION("The JsonVariant is a JsonObject") {
     DynamicJsonDocument doc;
-    JsonArray arr = doc.to<JsonArray>();
-    arr.add("hello");
-    JsonVariant var = arr;
-    var[0] = "world";
-    REQUIRE(1 == var.size());
-    REQUIRE(std::string("world") == var[0]);
-  }
+    JsonObject object = doc.to<JsonObject>();
+    JsonVariant var = object;
 
-  SECTION("NestedObjectSetValue") {
-    DynamicJsonDocument doc;
-    deserializeJson(doc, "[{}]");
-    JsonVariant var = doc.as<JsonVariant>();
-    var[0]["hello"] = "world";
-    REQUIRE(1 == var.size());
-    REQUIRE(1 == var[0].size());
-    REQUIRE(std::string("world") == var[0]["hello"]);
+    SECTION("get value") {
+      object["a"] = "element at key \"a\"";
+      object["b"] = "element at key \"b\"";
+
+      REQUIRE(2 == var.size());
+      REQUIRE(std::string("element at key \"a\"") == var["a"]);
+      REQUIRE(std::string("element at key \"b\"") == var["b"]);
+      REQUIRE(var["c"].isNull());
+      REQUIRE(var[0].isNull());
+    }
+
+    SECTION("set value, key is a const char*") {
+      var["hello"] = "world";
+
+      REQUIRE(1 == var.size());
+      REQUIRE(std::string("world") == var["hello"]);
+    }
+
+    SECTION("set value, key is a char[]") {
+      char key[] = "hello";
+      var[key] = "world";
+      key[0] = '!';  // make sure the key is duplicated
+
+      REQUIRE(1 == var.size());
+      REQUIRE(std::string("world") == var["hello"]);
+    }
   }
 }