diff --git a/test/cctest/test-debug.cc b/test/cctest/test-debug.cc
index d03785056d..b69ecccd6b 100644
--- a/test/cctest/test-debug.cc
+++ b/test/cctest/test-debug.cc
@@ -2202,8 +2202,9 @@ bool GetEvaluateStringResult(char *message, char* buffer, int buffer_size) {
   if (pos == NULL) {
     return false;
   }
-  Vector<char> buf(buffer, buffer_size - 1);
-  OS::StrNCpy(buf, pos, buffer_size);
+  Vector<char> buf(buffer, buffer_size);
+  OS::StrNCpy(buf, pos, buffer_size - 1);
+  buffer[buffer_size - 1] = '\0';
   return true;
 }