diff --git a/src/mem/metaslab.h b/src/mem/metaslab.h
index 1d24fae..c94de3e 100644
--- a/src/mem/metaslab.h
+++ b/src/mem/metaslab.h
@@ -57,11 +57,8 @@ namespace snmalloc
     // doubly linked node into that size class's free list.
     uint16_t link;
 
-    union
-    {
-      uint8_t sizeclass;
-      uint8_t next;
-    };
+    uint8_t sizeclass;
+    uint8_t next;
 
     void add_use()
     {
diff --git a/src/mem/superslab.h b/src/mem/superslab.h
index 5a1c901..fc2e4e8 100644
--- a/src/mem/superslab.h
+++ b/src/mem/superslab.h
@@ -194,6 +194,7 @@ namespace snmalloc
       uint8_t index = (uint8_t)slab_to_index(slab);
       uint8_t n = head - index - 1;
 
+      meta[index].sizeclass = 0;
       meta[index].next = n;
       head = index;
       bool was_almost_full = is_almost_full();