diff --git a/tools/releasetools/img_from_target_files.py b/tools/releasetools/img_from_target_files.py
index 6043355..9aab41c 100755
--- a/tools/releasetools/img_from_target_files.py
+++ b/tools/releasetools/img_from_target_files.py
@@ -154,8 +154,9 @@
 
   image_props = build_image.ImagePropFromGlobalDict(OPTIONS.info_dict,
                                                     "data")
-  # If no userdata_size is provided for extfs, skip userdata.img.
-  if (image_props.get("fs_type", "").startswith("ext") and
+  # We only allow yaffs to have a 0/missing partition_size.
+  # Extfs, f2fs must have a size. Skip userdata.img if no size.
+  if (not image_props.get("fs_type", "").startswith("yaffs") and
       not image_props.get("partition_size")):
     return