diff --git a/extras/tests/MixedConfiguration/progmem_emulation.hpp b/extras/tests/Helpers/progmem_emulation.hpp
similarity index 100%
rename from extras/tests/MixedConfiguration/progmem_emulation.hpp
rename to extras/tests/Helpers/progmem_emulation.hpp
diff --git a/extras/tests/Misc/StringAdapters.cpp b/extras/tests/Misc/StringAdapters.cpp
index ff4ee191..8e6fb6e5 100644
--- a/extras/tests/Misc/StringAdapters.cpp
+++ b/extras/tests/Misc/StringAdapters.cpp
@@ -2,9 +2,14 @@
 // Copyright Benoit Blanchon 2014-2020
 // MIT License
 
-#include <ArduinoJson.h>
-#include <catch.hpp>
 #include "custom_string.hpp"
+#include "progmem_emulation.hpp"
+
+#include <ArduinoJson/Strings/ConstRamStringAdapter.hpp>
+#include <ArduinoJson/Strings/FlashStringAdapter.hpp>
+#include <ArduinoJson/Strings/StlStringAdapter.hpp>
+
+#include <catch.hpp>
 
 using namespace ARDUINOJSON_NAMESPACE;
 
@@ -17,6 +22,8 @@ TEST_CASE("ConstRamStringAdapter") {
 
     REQUIRE(adapter.equals(NULL));
     REQUIRE_FALSE(adapter.equals("charlie"));
+
+    REQUIRE(adapter.size() == 0);
   }
 
   SECTION("non-null") {
@@ -29,6 +36,36 @@ TEST_CASE("ConstRamStringAdapter") {
 
     REQUIRE(adapter.equals("bravo"));
     REQUIRE_FALSE(adapter.equals("charlie"));
+
+    REQUIRE(adapter.size() == 5);
+  }
+}
+
+TEST_CASE("FlashStringAdapter") {
+  SECTION("null") {
+    FlashStringAdapter adapter(NULL);
+
+    REQUIRE(adapter.compare("bravo") < 0);
+    REQUIRE(adapter.compare(NULL) == 0);
+
+    REQUIRE(adapter.equals(NULL));
+    REQUIRE_FALSE(adapter.equals("charlie"));
+
+    REQUIRE(adapter.size() == 0);
+  }
+
+  SECTION("non-null") {
+    FlashStringAdapter adapter = adaptString(F("bravo"));
+
+    REQUIRE(adapter.compare(NULL) > 0);
+    REQUIRE(adapter.compare("alpha") > 0);
+    REQUIRE(adapter.compare("bravo") == 0);
+    REQUIRE(adapter.compare("charlie") < 0);
+
+    REQUIRE(adapter.equals("bravo"));
+    REQUIRE_FALSE(adapter.equals("charlie"));
+
+    REQUIRE(adapter.size() == 5);
   }
 }
 
@@ -43,6 +80,8 @@ TEST_CASE("std::string") {
 
   REQUIRE(adapter.equals("bravo"));
   REQUIRE_FALSE(adapter.equals("charlie"));
+
+  REQUIRE(adapter.size() == 5);
 }
 
 TEST_CASE("custom_string") {
@@ -56,6 +95,8 @@ TEST_CASE("custom_string") {
 
   REQUIRE(adapter.equals("bravo"));
   REQUIRE_FALSE(adapter.equals("charlie"));
+
+  REQUIRE(adapter.size() == 5);
 }
 
 TEST_CASE("IsString<T>") {
@@ -70,4 +111,8 @@ TEST_CASE("IsString<T>") {
   SECTION("custom_string") {
     REQUIRE(IsString<custom_string>::value == true);
   }
+
+  SECTION("const __FlashStringHelper*") {
+    REQUIRE(IsString<const __FlashStringHelper*>::value == true);
+  }
 }
diff --git a/extras/tests/Misc/custom_string.hpp b/extras/tests/Misc/custom_string.hpp
index 2bbbd2f3..8ba892b0 100644
--- a/extras/tests/Misc/custom_string.hpp
+++ b/extras/tests/Misc/custom_string.hpp
@@ -6,8 +6,6 @@
 
 #include <string>
 
-using namespace ARDUINOJSON_NAMESPACE;
-
 struct custom_char_traits : std::char_traits<char> {};
 struct custom_allocator : std::allocator<char> {};
 typedef std::basic_string<char, custom_char_traits, custom_allocator>
diff --git a/src/ArduinoJson/Polyfills/pgmspace.hpp b/src/ArduinoJson/Polyfills/pgmspace.hpp
index 85997188..ffeffd18 100644
--- a/src/ArduinoJson/Polyfills/pgmspace.hpp
+++ b/src/ArduinoJson/Polyfills/pgmspace.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Namespace.hpp>
+#include <ArduinoJson/Polyfills/assert.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
 // Wraps a const char* so that the our functions are picked only if the
@@ -18,6 +19,7 @@ struct pgm_p {
 #ifndef strlen_P
 inline size_t strlen_P(ARDUINOJSON_NAMESPACE::pgm_p s) {
   const char* p = s.address;
+  ARDUINOJSON_ASSERT(p != NULL);
   while (pgm_read_byte(p)) p++;
   return size_t(p - s.address);
 }
@@ -27,6 +29,8 @@ inline size_t strlen_P(ARDUINOJSON_NAMESPACE::pgm_p s) {
 inline int strncmp_P(const char* a, ARDUINOJSON_NAMESPACE::pgm_p b, size_t n) {
   const char* s1 = a;
   const char* s2 = b.address;
+  ARDUINOJSON_ASSERT(s1 != NULL);
+  ARDUINOJSON_ASSERT(s2 != NULL);
   while (n-- > 0) {
     char c1 = *s1++;
     char c2 = static_cast<char>(pgm_read_byte(s2++));
@@ -45,6 +49,8 @@ inline int strncmp_P(const char* a, ARDUINOJSON_NAMESPACE::pgm_p b, size_t n) {
 inline int strcmp_P(const char* a, ARDUINOJSON_NAMESPACE::pgm_p b) {
   const char* s1 = a;
   const char* s2 = b.address;
+  ARDUINOJSON_ASSERT(s1 != NULL);
+  ARDUINOJSON_ASSERT(s2 != NULL);
   for (;;) {
     char c1 = *s1++;
     char c2 = static_cast<char>(pgm_read_byte(s2++));
@@ -62,6 +68,8 @@ inline int strcmp_P(const char* a, ARDUINOJSON_NAMESPACE::pgm_p b) {
 inline void* memcpy_P(void* dst, ARDUINOJSON_NAMESPACE::pgm_p src, size_t n) {
   uint8_t* d = reinterpret_cast<uint8_t*>(dst);
   const char* s = src.address;
+  ARDUINOJSON_ASSERT(d != NULL);
+  ARDUINOJSON_ASSERT(s != NULL);
   while (n-- > 0) {
     *d++ = pgm_read_byte(s++);
   }
diff --git a/src/ArduinoJson/Polyfills/safe_strcmp.hpp b/src/ArduinoJson/Polyfills/safe_strcmp.hpp
index 2c5f7d89..135f4957 100644
--- a/src/ArduinoJson/Polyfills/safe_strcmp.hpp
+++ b/src/ArduinoJson/Polyfills/safe_strcmp.hpp
@@ -6,6 +6,8 @@
 
 #include <ArduinoJson/Namespace.hpp>
 
+#include <stdint.h>  // int8_t
+
 namespace ARDUINOJSON_NAMESPACE {
 
 inline int8_t safe_strcmp(const char* a, const char* b) {
diff --git a/src/ArduinoJson/Strings/ArduinoStringAdapter.hpp b/src/ArduinoJson/Strings/ArduinoStringAdapter.hpp
index ac7bbdc3..c2e39d84 100644
--- a/src/ArduinoJson/Strings/ArduinoStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/ArduinoStringAdapter.hpp
@@ -7,6 +7,7 @@
 #include <WString.h>
 
 #include <ArduinoJson/Polyfills/safe_strcmp.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/ConstRamStringAdapter.hpp b/src/ArduinoJson/Strings/ConstRamStringAdapter.hpp
index 407f584d..7cc95390 100644
--- a/src/ArduinoJson/Strings/ConstRamStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/ConstRamStringAdapter.hpp
@@ -8,6 +8,7 @@
 #include <string.h>  // strcmp
 
 #include <ArduinoJson/Polyfills/safe_strcmp.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/FlashStringAdapter.hpp b/src/ArduinoJson/Strings/FlashStringAdapter.hpp
index 328f040f..673a2b31 100644
--- a/src/ArduinoJson/Strings/FlashStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/FlashStringAdapter.hpp
@@ -4,7 +4,9 @@
 
 #pragma once
 
+#include <ArduinoJson/Memory/MemoryPool.hpp>
 #include <ArduinoJson/Polyfills/pgmspace.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/IsString.hpp b/src/ArduinoJson/Strings/IsString.hpp
new file mode 100644
index 00000000..18cbea85
--- /dev/null
+++ b/src/ArduinoJson/Strings/IsString.hpp
@@ -0,0 +1,18 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+#include <ArduinoJson/Polyfills/type_traits.hpp>
+
+namespace ARDUINOJSON_NAMESPACE {
+template <typename>
+struct IsString : false_type {};
+
+template <typename T>
+struct IsString<const T> : IsString<T> {};
+
+template <typename T>
+struct IsString<T&> : IsString<T> {};
+}  // namespace ARDUINOJSON_NAMESPACE
diff --git a/src/ArduinoJson/Strings/RamStringAdapter.hpp b/src/ArduinoJson/Strings/RamStringAdapter.hpp
index 05db2e84..9a80c0aa 100644
--- a/src/ArduinoJson/Strings/RamStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/RamStringAdapter.hpp
@@ -4,7 +4,9 @@
 
 #pragma once
 
+#include <ArduinoJson/Memory/MemoryPool.hpp>
 #include <ArduinoJson/Strings/ConstRamStringAdapter.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp b/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
index 41a6cf6d..2d77b390 100644
--- a/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Namespace.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/SizedRamStringAdapter.hpp b/src/ArduinoJson/Strings/SizedRamStringAdapter.hpp
index 911f4af6..c54769fe 100644
--- a/src/ArduinoJson/Strings/SizedRamStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/SizedRamStringAdapter.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Namespace.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 #include <string.h>  // strcmp
diff --git a/src/ArduinoJson/Strings/StlStringAdapter.hpp b/src/ArduinoJson/Strings/StlStringAdapter.hpp
index defec413..c5eef2be 100644
--- a/src/ArduinoJson/Strings/StlStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/StlStringAdapter.hpp
@@ -4,7 +4,9 @@
 
 #pragma once
 
+#include <ArduinoJson/Memory/MemoryPool.hpp>
 #include <ArduinoJson/Namespace.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 #include <string>
diff --git a/src/ArduinoJson/Strings/String.hpp b/src/ArduinoJson/Strings/String.hpp
index 95e4f79a..c6f16baf 100644
--- a/src/ArduinoJson/Strings/String.hpp
+++ b/src/ArduinoJson/Strings/String.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/Strings/ConstRamStringAdapter.hpp>
+#include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StoragePolicy.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
diff --git a/src/ArduinoJson/Strings/StringAdapters.hpp b/src/ArduinoJson/Strings/StringAdapters.hpp
index db9f9e48..30b45d04 100644
--- a/src/ArduinoJson/Strings/StringAdapters.hpp
+++ b/src/ArduinoJson/Strings/StringAdapters.hpp
@@ -4,20 +4,6 @@
 
 #pragma once
 
-#include <ArduinoJson/Memory/MemoryPool.hpp>
-#include <ArduinoJson/Polyfills/type_traits.hpp>
-
-namespace ARDUINOJSON_NAMESPACE {
-template <typename>
-struct IsString : false_type {};
-
-template <typename T>
-struct IsString<const T> : IsString<T> {};
-
-template <typename T>
-struct IsString<T&> : IsString<T> {};
-}  // namespace ARDUINOJSON_NAMESPACE
-
 #include <ArduinoJson/Strings/ConstRamStringAdapter.hpp>
 #include <ArduinoJson/Strings/RamStringAdapter.hpp>
 #include <ArduinoJson/Strings/SizedRamStringAdapter.hpp>