diff --git a/pngwutil.c b/pngwutil.c
index 231e5d32d..2674b9fb8 100644
--- a/pngwutil.c
+++ b/pngwutil.c
@@ -482,10 +482,10 @@ png_zlib_compress_validate(png_zlib_compressp pz, int in_use)
 
    if (pz->overflow == 0U && pz->len == 0U && pz->start == 0U) /* empty */
    {
-      affirm((pz->end == &pz->list->next && pz->zs.next_out == pz->list->output
-               && pz->zs.avail_out == o_size) ||
-             (pz->end == &pz->list && pz->zs.next_out == NULL
-               && pz->zs.avail_out == 0U));
+      affirm((pz->end == &pz->list && pz->zs.next_out == NULL
+              && pz->zs.avail_out == 0U) ||
+             (pz->end == &pz->list->next && pz->zs.next_out == pz->list->output
+              && pz->zs.avail_out == o_size));
    }
 
    else /* not empty */