diff --git a/core-implement/HardwareSerial.cpp b/core-implement/HardwareSerial.cpp
index 67b2e33..ff1327f 100644
--- a/core-implement/HardwareSerial.cpp
+++ b/core-implement/HardwareSerial.cpp
@@ -150,7 +150,8 @@ int UART::printf(const char *format, ...){
     va_list args;
     va_start(args, format);
     const int space = vsnprintf(NULL, 0, format, args) + 1;
-    char buf[space];
+    char *buf;
+    buf = (char *) alloca(space);
     memset(buf, 0x00, space);
     vsnprintf(buf, space, format, args);
     va_end(args);