diff --git a/stdlib/public/runtime/ReflectionMirror.mm b/stdlib/public/runtime/ReflectionMirror.mm
index 74a644912e19..b24737360dc5 100644
--- a/stdlib/public/runtime/ReflectionMirror.mm
+++ b/stdlib/public/runtime/ReflectionMirror.mm
@@ -56,6 +56,7 @@ int asprintf(char **strp, const char *fmt, ...) {
     return -1;
 
   length = _vsnprintf(*strp, length, fmt, argp1);
+  (*strp)[length] = '\0';
 
   va_end(argp0);
   va_end(argp1);