diff --git a/src/libostree/ostree-repo-commit.c b/src/libostree/ostree-repo-commit.c
index 8f059e11..752a01be 100644
--- a/src/libostree/ostree-repo-commit.c
+++ b/src/libostree/ostree-repo-commit.c
@@ -352,7 +352,13 @@ repo_setup_generate_sizes (OstreeRepo               *self,
   if (modifier && modifier->flags & OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES)
     {
       if (ostree_repo_get_mode (self) == OSTREE_REPO_MODE_ARCHIVE)
-        self->generate_sizes = TRUE;
+        {
+          self->generate_sizes = TRUE;
+
+          /* Clear any stale data in the object sizes hash table */
+          if (self->object_sizes != NULL)
+            g_hash_table_remove_all (self->object_sizes);
+        }
       else
         g_debug ("Not generating sizes for non-archive repo");
     }
@@ -428,6 +434,9 @@ add_size_index_to_metadata (OstreeRepo        *self,
 
       g_variant_builder_add (builder, "{sv}", "ostree.sizes",
                              g_variant_builder_end (&index_builder));
+
+      /* Clear the object sizes hash table for a subsequent commit. */
+      g_hash_table_remove_all (self->object_sizes);
     }
 
   return g_variant_ref_sink (g_variant_builder_end (builder));
diff --git a/src/libostree/ostree-repo-private.h b/src/libostree/ostree-repo-private.h
index b57ad799..bc2325e5 100644
--- a/src/libostree/ostree-repo-private.h
+++ b/src/libostree/ostree-repo-private.h
@@ -143,6 +143,14 @@ struct OstreeRepo {
   guint zlib_compression_level;
   GHashTable *loose_object_devino_hash;
   GHashTable *updated_uncompressed_dirs;
+
+  /* FIXME: The object sizes hash table is really per-commit state, not repo
+   * state. Using a single table for the repo means that commits cannot be
+   * built simultaneously if they're adding size information. This data should
+   * probably be in OstreeMutableTree, but that's gone by the time the actual
+   * commit is constructed. At that point the only commit state is in the root
+   * OstreeRepoFile.
+   */
   GHashTable *object_sizes;
 
   /* Cache the repo's device/inode to use for comparisons elsewhere */