diff --git a/src/bibx/__init__.py b/src/bibx/__init__.py
index 1a64d3e..4b8d583 100644
--- a/src/bibx/__init__.py
+++ b/src/bibx/__init__.py
@@ -28,7 +28,7 @@
     "read_wos",
 ]
 
-__version__ = "0.6.2"
+__version__ = "0.6.3"
 
 
 def query_openalex(
diff --git a/src/bibx/builders/wos.py b/src/bibx/builders/wos.py
index bcd40fb..43273ac 100644
--- a/src/bibx/builders/wos.py
+++ b/src/bibx/builders/wos.py
@@ -61,7 +61,7 @@ class WosCollectionBuilder(CollectionBuilder):
     """Builder for collections of articles from Web of Science (WoS) ISI files."""
 
     ISI_LINE_PATTERN = re.compile(
-        r"^(null|.)?((?P<field>[A-Z0-9]{2})|  )( (?P<value>.*))?$"
+        r"^((null)+|.)?((?P<field>[A-Z0-9]{2})|  )( (?P<value>.*))?$"
     )
     ISI_CITATION_PATTERN = re.compile(
         r"""^(?P<AU>[^,]+),[ ]          # First author