diff --git a/hardware/arduino/avr/cores/arduino/WString.cpp b/hardware/arduino/avr/cores/arduino/WString.cpp
index ace128dad..8c85cc592 100644
--- a/hardware/arduino/avr/cores/arduino/WString.cpp
+++ b/hardware/arduino/avr/cores/arduino/WString.cpp
@@ -296,14 +296,14 @@ unsigned char String::concat(unsigned char num)
 
 unsigned char String::concat(int num)
 {
-	char buf[7];
+	char buf[12];
 	itoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }
 
 unsigned char String::concat(unsigned int num)
 {
-	char buf[6];
+	char buf[11];
 	utoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }
diff --git a/hardware/arduino/avr/cores/robot/WString.cpp b/hardware/arduino/avr/cores/robot/WString.cpp
index ace128dad..8c85cc592 100644
--- a/hardware/arduino/avr/cores/robot/WString.cpp
+++ b/hardware/arduino/avr/cores/robot/WString.cpp
@@ -296,14 +296,14 @@ unsigned char String::concat(unsigned char num)
 
 unsigned char String::concat(int num)
 {
-	char buf[7];
+	char buf[12];
 	itoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }
 
 unsigned char String::concat(unsigned int num)
 {
-	char buf[6];
+	char buf[11];
 	utoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }
diff --git a/hardware/arduino/sam/cores/arduino/WString.cpp b/hardware/arduino/sam/cores/arduino/WString.cpp
index fa47491a9..de1a2f1b2 100644
--- a/hardware/arduino/sam/cores/arduino/WString.cpp
+++ b/hardware/arduino/sam/cores/arduino/WString.cpp
@@ -297,14 +297,14 @@ unsigned char String::concat(unsigned char num)
 
 unsigned char String::concat(int num)
 {
-	char buf[7];
+	char buf[12];
 	itoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }
 
 unsigned char String::concat(unsigned int num)
 {
-	char buf[6];
+	char buf[11];
 	utoa(num, buf, 10);
 	return concat(buf, strlen(buf));
 }