diff --git a/src/physfs_archiver_zip.c b/src/physfs_archiver_zip.c
index c82eeb0..f18acec 100644
--- a/src/physfs_archiver_zip.c
+++ b/src/physfs_archiver_zip.c
@@ -1497,7 +1497,7 @@ static PHYSFS_Io *zip_get_io(PHYSFS_Io *io, ZIPinfo *inf, ZIPentry *entry)
     PHYSFS_Io *retval = io->duplicate(io);
     BAIL_IF_ERRPASS(!retval, NULL);
 
-    /* !!! FIXME: if you open a dir here, it should bail ERR_NOT_A_FILE */
+    assert(!entry->tree.isdir); /* should have been checked before calling. */
 
     /* (inf) can be NULL if we already resolved. */
     success = (inf == NULL) || zip_resolve(retval, inf, entry);