diff --git a/extras/tests/TextFormatter/writeString.cpp b/extras/tests/TextFormatter/writeString.cpp
index b36cf666..8341b6cb 100644
--- a/extras/tests/TextFormatter/writeString.cpp
+++ b/extras/tests/TextFormatter/writeString.cpp
@@ -19,10 +19,6 @@ void check(const char* input, std::string expected) {
 }
 
 TEST_CASE("TextFormatter::writeString()") {
-  SECTION("Null") {
-    check(0, "null");
-  }
-
   SECTION("EmptyString") {
     check("", "\"\"");
   }
diff --git a/src/ArduinoJson/Json/TextFormatter.hpp b/src/ArduinoJson/Json/TextFormatter.hpp
index a1ef2899..6e0ca70d 100644
--- a/src/ArduinoJson/Json/TextFormatter.hpp
+++ b/src/ArduinoJson/Json/TextFormatter.hpp
@@ -10,6 +10,7 @@
 #include <ArduinoJson/Json/EscapeSequence.hpp>
 #include <ArduinoJson/Numbers/FloatParts.hpp>
 #include <ArduinoJson/Numbers/Integer.hpp>
+#include <ArduinoJson/Polyfills/assert.hpp>
 #include <ArduinoJson/Polyfills/attributes.hpp>
 
 namespace ARDUINOJSON_NAMESPACE {
@@ -32,13 +33,10 @@ class TextFormatter {
   }
 
   void writeString(const char *value) {
-    if (!value) {
-      writeRaw("null");
-    } else {
-      writeRaw('\"');
-      while (*value) writeChar(*value++);
-      writeRaw('\"');
-    }
+    ARDUINOJSON_ASSERT(value != NULL);
+    writeRaw('\"');
+    while (*value) writeChar(*value++);
+    writeRaw('\"');
   }
 
   void writeChar(char c) {
diff --git a/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp b/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
index d239f501..eedff5b4 100644
--- a/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
+++ b/src/ArduinoJson/MsgPack/MsgPackSerializer.hpp
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <ArduinoJson/MsgPack/endianess.hpp>
+#include <ArduinoJson/Polyfills/assert.hpp>
 #include <ArduinoJson/Polyfills/type_traits.hpp>
 #include <ArduinoJson/Serialization/measure.hpp>
 #include <ArduinoJson/Serialization/serialize.hpp>
@@ -70,8 +71,7 @@ class MsgPackSerializer {
   }
 
   void visitString(const char* value) {
-    if (!value)
-      return writeByte(0xC0);  // nil
+    ARDUINOJSON_ASSERT(value != NULL);
 
     size_t n = strlen(value);