diff --git a/gtk/timsort/gtktimsort.c b/gtk/timsort/gtktimsort.c
index 67846674e2..daa7c113db 100644
--- a/gtk/timsort/gtktimsort.c
+++ b/gtk/timsort/gtktimsort.c
@@ -157,8 +157,9 @@ gtk_tim_sort_ensure_capacity (GtkTimSort *self,
       new_size |= new_size >> 4;
       new_size |= new_size >> 8;
       new_size |= new_size >> 16;
-      if (sizeof(new_size) > 4)
-        new_size |= new_size >> 32;
+#if GLIB_SIZEOF_SIZE_T > 4
+      new_size |= new_size >> 32;
+#endif
 
       new_size++;
       if (new_size == 0) /* (overflow) Not bloody likely! */