diff --git a/src/maxdiffusion/models/attention_flax.py b/src/maxdiffusion/models/attention_flax.py
index 3572c2b8..edc9f4f7 100644
--- a/src/maxdiffusion/models/attention_flax.py
+++ b/src/maxdiffusion/models/attention_flax.py
@@ -635,19 +635,19 @@ def wrap_ulysses_attention(query, key, value):
 
       if flash_block_sizes is not None:
         if isinstance(flash_block_sizes, dict):
-          bq = flash_block_sizes.get("block_q", bq)
-          bkv = flash_block_sizes.get("block_kv", bkv)
-          bkv_compute = flash_block_sizes.get("block_kv_compute", bkv_compute)
-          bkv_compute_in = flash_block_sizes.get("block_kv_compute_in", bkv_compute_in)
-          heads_per_tile = flash_block_sizes.get("heads_per_tile", heads_per_tile)
-          vmem_limit_bytes = flash_block_sizes.get("vmem_limit_bytes", vmem_limit_bytes)
+          bq = flash_block_sizes.get("block_q", None) or bq
+          bkv = flash_block_sizes.get("block_kv", None) or bkv
+          bkv_compute = flash_block_sizes.get("block_kv_compute", None) or bkv_compute
+          bkv_compute_in = flash_block_sizes.get("block_kv_compute_in", None) or bkv_compute_in
+          heads_per_tile = flash_block_sizes.get("heads_per_tile", None) or heads_per_tile
+          vmem_limit_bytes = flash_block_sizes.get("vmem_limit_bytes", None) or vmem_limit_bytes
         else:
-          bq = getattr(flash_block_sizes, "block_q", bq)
-          bkv = getattr(flash_block_sizes, "block_kv", bkv)
-          bkv_compute = getattr(flash_block_sizes, "block_kv_compute", bkv_compute)
-          bkv_compute_in = getattr(flash_block_sizes, "block_kv_compute_in", bkv_compute_in)
-          heads_per_tile = getattr(flash_block_sizes, "heads_per_tile", heads_per_tile)
-          vmem_limit_bytes = getattr(flash_block_sizes, "vmem_limit_bytes", vmem_limit_bytes)
+          bq = getattr(flash_block_sizes, "block_q", None) or bq
+          bkv = getattr(flash_block_sizes, "block_kv", None) or bkv
+          bkv_compute = getattr(flash_block_sizes, "block_kv_compute", None) or bkv_compute
+          bkv_compute_in = getattr(flash_block_sizes, "block_kv_compute_in", None) or bkv_compute_in
+          heads_per_tile = getattr(flash_block_sizes, "heads_per_tile", None) or heads_per_tile
+          vmem_limit_bytes = getattr(flash_block_sizes, "vmem_limit_bytes", None) or vmem_limit_bytes
 
       if use_base2_exp:
         query = query * LOG2E