diff --git a/cores/esp32/WString.cpp b/cores/esp32/WString.cpp
index 5296a2d9652..18e64767545 100644
--- a/cores/esp32/WString.cpp
+++ b/cores/esp32/WString.cpp
@@ -290,7 +290,8 @@ String &String::operator=(StringSumHelper &&rval) {
 #endif
 
 String &String::operator=(const char *cstr) {
-  return copy(cstr, strlen(cstr));
+  const uint32_t length = cstr ? strlen(cstr) : 0u;
+  return copy(cstr, length);
 }
 
 /*********************************************/