diff --git a/cores/arduino/WString.cpp b/cores/arduino/WString.cpp
index 73a96f9f..ce4e4f85 100644
--- a/cores/arduino/WString.cpp
+++ b/cores/arduino/WString.cpp
@@ -227,7 +227,7 @@ String & String::copy(const __FlashStringHelper *pstr, unsigned int length)
 void String::move(String &rhs)
 {
 	if (buffer) {
-		if (capacity >= rhs.len) {
+		if (rhs && capacity >= rhs.len) {
 			strcpy(buffer, rhs.buffer);
 			len = rhs.len;
 			rhs.len = 0;