diff --git a/memtag.h b/memtag.h
index fec258b..26502e2 100644
--- a/memtag.h
+++ b/memtag.h
@@ -158,6 +158,7 @@
 };
 
 inline uptr selectRandomTag(uptr Ptr, uptr ExcludeMask) {
+  ExcludeMask |= 1; // Always exclude Tag 0.
   uptr TaggedPtr;
   __asm__ __volatile__(
       R"(