diff --git a/src/ArduinoJson/Namespace.hpp b/src/ArduinoJson/Namespace.hpp
index 76c4dcf9..8108c5da 100644
--- a/src/ArduinoJson/Namespace.hpp
+++ b/src/ArduinoJson/Namespace.hpp
@@ -10,19 +10,18 @@
 
 #define ARDUINOJSON_DO_CONCAT(A, B) A##B
 #define ARDUINOJSON_CONCAT2(A, B) ARDUINOJSON_DO_CONCAT(A, B)
-#define ARDUINOJSON_CONCAT3(A, B, C) \
-  ARDUINOJSON_CONCAT2(A, ARDUINOJSON_CONCAT2(B, C))
 #define ARDUINOJSON_CONCAT4(A, B, C, D) \
   ARDUINOJSON_CONCAT2(ARDUINOJSON_CONCAT2(A, B), ARDUINOJSON_CONCAT2(C, D))
 #define ARDUINOJSON_CONCAT8(A, B, C, D, E, F, G, H)    \
   ARDUINOJSON_CONCAT2(ARDUINOJSON_CONCAT4(A, B, C, D), \
                       ARDUINOJSON_CONCAT4(E, F, G, H))
-#define ARDUINOJSON_CONCAT10(A, B, C, D, E, F, G, H, I, J) \
-  ARDUINOJSON_CONCAT8(A, B, C, D, E, F, G, ARDUINOJSON_CONCAT3(H, I, J))
+#define ARDUINOJSON_CONCAT11(A, B, C, D, E, F, G, H, I, J, K) \
+  ARDUINOJSON_CONCAT8(A, B, C, D, E, F, G, ARDUINOJSON_CONCAT4(H, I, J, K))
 
 #define ARDUINOJSON_NAMESPACE                                            \
-  ARDUINOJSON_CONCAT10(                                                  \
+  ARDUINOJSON_CONCAT11(                                                  \
       ArduinoJson, ARDUINOJSON_VERSION_MAJOR, ARDUINOJSON_VERSION_MINOR, \
       ARDUINOJSON_VERSION_REVISION, _, ARDUINOJSON_USE_LONG_LONG,        \
       ARDUINOJSON_USE_DOUBLE, ARDUINOJSON_DECODE_UNICODE,                \
-      ARDUINOJSON_ENABLE_NAN, ARDUINOJSON_ENABLE_INFINITY)
+      ARDUINOJSON_ENABLE_NAN, ARDUINOJSON_ENABLE_INFINITY,               \
+      ARDUINOJSON_ENABLE_PROGMEM)
diff --git a/src/ArduinoJson/Strings/FlashStringAdapter.hpp b/src/ArduinoJson/Strings/FlashStringAdapter.hpp
index bb0fdd68..d1b094ad 100644
--- a/src/ArduinoJson/Strings/FlashStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/FlashStringAdapter.hpp
@@ -14,7 +14,7 @@ class FlashStringAdapter {
     if (!other && !_str) return 0;
     if (!_str) return -1;
     if (!other) return 1;
-    return -strcmp_P(other, reinterpret_cast<const char*>(_str));
+    return int8_t(-strcmp_P(other, reinterpret_cast<const char*>(_str)));
   }
 
   bool equals(const char* expected) const {
diff --git a/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp b/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
index 323078af..a444c5f6 100644
--- a/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/SizedFlashStringAdapter.hpp
@@ -15,7 +15,8 @@ class SizedFlashStringAdapter {
     if (!other && !_str) return 0;
     if (!_str) return -1;
     if (!other) return 1;
-    return -strncmp_P(other, reinterpret_cast<const char*>(_str), _size);
+    return int8_t(
+        -strncmp_P(other, reinterpret_cast<const char*>(_str), _size));
   }
 
   bool equals(const char* expected) const {
@@ -29,7 +30,7 @@ class SizedFlashStringAdapter {
   char* save(MemoryPool* pool) const {
     if (!_str) return NULL;
     char* dup = pool->allocFrozenString(_size);
-    if (dup) memcpy_P(dup, (const char*)_str, _size);
+    if (dup) memcpy_P(dup, reinterpret_cast<const char*>(_str), _size);
     return dup;
   }
 
diff --git a/test/MixedConfiguration/CMakeLists.txt b/test/MixedConfiguration/CMakeLists.txt
index 038317d3..afcf514b 100644
--- a/test/MixedConfiguration/CMakeLists.txt
+++ b/test/MixedConfiguration/CMakeLists.txt
@@ -17,6 +17,7 @@ add_executable(MixedConfigurationTests
 	use_double_1.cpp
 	use_long_long_0.cpp
 	use_long_long_1.cpp
+	enable_progmem_1.cpp
 )
 
 target_link_libraries(MixedConfigurationTests catch)
diff --git a/test/MixedConfiguration/enable_progmem_1.cpp b/test/MixedConfiguration/enable_progmem_1.cpp
new file mode 100644
index 00000000..187ec30a
--- /dev/null
+++ b/test/MixedConfiguration/enable_progmem_1.cpp
@@ -0,0 +1,53 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2019
+// MIT License
+
+#include "progmem_emulation.hpp"
+
+#define ARDUINOJSON_ENABLE_PROGMEM 1
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+
+TEST_CASE("Flash strings") {
+  DynamicJsonDocument doc(2048);
+
+  SECTION("deserializeJson()") {
+    DeserializationError err = deserializeJson(doc, F("{'hello':'world'}"));
+
+    REQUIRE(err == DeserializationError::Ok);
+    REQUIRE(doc["hello"] == "world");
+  }
+
+  SECTION("JsonDocument::operator[]") {
+    doc[F("hello")] = F("world");
+
+    REQUIRE(doc["hello"] == "world");
+  }
+
+  SECTION("JsonDocument::add()") {
+    doc.add(F("world"));
+
+    REQUIRE(doc[0] == "world");
+  }
+
+  SECTION("JsonVariant::set()") {
+    JsonVariant var = doc.to<JsonVariant>();
+
+    var.set(F("world"));
+
+    REQUIRE(var == "world");
+  }
+
+  SECTION("MemberProxy::operator==") {
+    doc["hello"] = "world";
+
+    REQUIRE(doc["hello"] == F("world"));
+  }
+
+  SECTION("ElementProxy::operator==") {
+    doc.add("world");
+
+    REQUIRE(doc[0] == F("world"));
+  }
+}
diff --git a/test/MixedConfiguration/progmem_emulation.hpp b/test/MixedConfiguration/progmem_emulation.hpp
new file mode 100644
index 00000000..b0bc8b50
--- /dev/null
+++ b/test/MixedConfiguration/progmem_emulation.hpp
@@ -0,0 +1,41 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2019
+// MIT License
+
+#include <stdint.h>  // uint8_t
+#include <string.h>  // strcmp, strlen...
+
+class __FlashStringHelper;
+
+typedef char prog_char;
+typedef void prog_void;
+
+inline const void* convertPtrToFlash(const void* s) {
+  return reinterpret_cast<const char*>(s) + 42;
+}
+
+inline const void* convertFlashToPtr(const void* s) {
+  return reinterpret_cast<const char*>(s) - 42;
+}
+
+#define F(X) reinterpret_cast<const __FlashStringHelper*>(convertPtrToFlash(X))
+
+inline uint8_t pgm_read_byte_near(const void* p) {
+  return *reinterpret_cast<const uint8_t*>(convertFlashToPtr(p));
+}
+
+inline void* memcpy_P(void* a, const prog_void* b, size_t n) {
+  return memcpy(a, convertFlashToPtr(b), n);
+}
+
+inline int strcmp_P(const char* a, const prog_char* b) {
+  return strcmp(a, reinterpret_cast<const char*>(convertFlashToPtr(b)));
+}
+
+inline int strncmp_P(const char* a, const prog_char* b, size_t n) {
+  return strncmp(a, reinterpret_cast<const char*>(convertFlashToPtr(b)), n);
+}
+
+inline size_t strlen_P(const prog_char* s) {
+  return strlen(reinterpret_cast<const char*>(convertFlashToPtr(s)));
+}