diff --git a/libraries/AP_HAL/Util.cpp b/libraries/AP_HAL/Util.cpp
index 4b67df77d5..8c66a01976 100644
--- a/libraries/AP_HAL/Util.cpp
+++ b/libraries/AP_HAL/Util.cpp
@@ -52,9 +52,12 @@ int AP_HAL::Util::snprintf(char* str, size_t size, const char *format, ...)
 
 int AP_HAL::Util::vsnprintf(char* str, size_t size, const char *format, va_list ap)
 {
-    BufferPrinter buf(str, size);
+    if (size == 0) {
+        return 0;
+    }
+    BufferPrinter buf(str, size-1);
     print_vprintf(&buf, format, ap);
-    // null terminate if possible
+    // null terminate
     int ret = buf._offs;
     buf.write(0);
     return ret;