diff --git a/pyscf/dft/numint.py b/pyscf/dft/numint.py
index 9947ed5c30..0b3cfdc74c 100644
--- a/pyscf/dft/numint.py
+++ b/pyscf/dft/numint.py
@@ -2802,7 +2802,7 @@ def block_loop(self, mol, grids, nao=None, deriv=0, max_memory=2000,
         screen_index = non0tab
 
         # the xxx_sparse() functions require ngrids 8-byte aligned
-        allow_sparse = ngrids % ALIGNMENT_UNIT == 0
+        allow_sparse = ngrids % ALIGNMENT_UNIT == 0 and nao > SWITCH_SIZE
 
         if buf is None:
             buf = _empty_aligned(comp * blksize * nao)