diff --git a/src/Zcu.zig b/src/Zcu.zig
index d26417c8f7..523c14251e 100644
--- a/src/Zcu.zig
+++ b/src/Zcu.zig
@@ -2932,6 +2932,12 @@ pub fn saveZirCache(gpa: Allocator, cache_file: std.fs.File, stat: std.fs.File.S
             const as_struct: *const HackDataLayout = @ptrCast(data);
             safety_buffer[i] = as_struct.data;
         }
+        // Union variants smaller than 8 bytes leave padding uninitialised.
+        // The copy above propagates those V-bits into safety_buffer, so
+        // valgrind flags the subsequent pwritev. The padding is never
+        // interpreted on read-back (the tag array determines the active
+        // variant), so tell valgrind the buffer contents are intentional.
+        std.valgrind.memcheck.makeMemDefined(std.mem.sliceAsBytes(safety_buffer));
     }
 
     const header: Zir.Header = .{