diff --git a/src/mods4pandas/lib.py b/src/mods4pandas/lib.py
index 32f717a..cff8ea9 100644
--- a/src/mods4pandas/lib.py
+++ b/src/mods4pandas/lib.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from itertools import groupby
 import re
 import warnings
@@ -24,40 +26,40 @@ ns = {
 class TagGroup:
     """Helper class to simplify the parsing and checking of MODS metadata"""
 
-    def __init__(self, tag, group: List[ET.Element]):
+    def __init__(self, tag, group: List[ET._Element]):
         self.tag = tag
         self.group = group
 
-    def to_xml(self):
+    def to_xml(self) -> str:
         return '\n'.join(str(ET.tostring(e), 'utf-8').strip() for e in self.group)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return f"TagGroup with content:\n{self.to_xml()}"
 
-    def is_singleton(self):
+    def is_singleton(self) -> TagGroup:
         if len(self.group) != 1:
             raise ValueError('More than one instance: {}'.format(self))
         return self
 
-    def has_no_attributes(self):
+    def has_no_attributes(self) -> TagGroup:
         return self.has_attributes({})
 
-    def has_attributes(self, attrib):
+    def has_attributes(self, attrib) -> TagGroup:
         if not isinstance(attrib, Sequence):
             attrib = [attrib]
         if not all(e.attrib in attrib for e in self.group):
             raise ValueError('One or more element has unexpected attributes: {}'.format(self))
         return self
 
-    def ignore_attributes(self):
+    def ignore_attributes(self) -> TagGroup:
         # This serves as documentation for now.
         return self
 
-    def sort(self, key=None, reverse=False):
+    def sort(self, key=None, reverse=False) -> TagGroup:
         self.group = sorted(self.group, key=key, reverse=reverse)
         return self
 
-    def text(self, separator='\n'):
+    def text(self, separator='\n') -> str:
         t = ''
         for e in self.group:
             if t != '':
@@ -66,13 +68,13 @@ class TagGroup:
                 t += e.text
         return t
 
-    def text_set(self):
+    def text_set(self) -> set:
         return {e.text for e in self.group}
 
-    def descend(self, raise_errors):
+    def descend(self, raise_errors) -> dict:
         return _to_dict(self.is_singleton().group[0], raise_errors)
 
-    def filter(self, cond, warn=None):
+    def filter(self, cond, warn=None) -> TagGroup:
         new_group = []
         for e in self.group:
             if cond(e):
@@ -82,7 +84,7 @@ class TagGroup:
                     warnings.warn('Filtered {} element ({})'.format(self.tag, warn))
         return TagGroup(self.tag, new_group)
 
-    def force_singleton(self, warn=True):
+    def force_singleton(self, warn=True) -> TagGroup:
         if len(self.group) == 1:
             return self
         else:
@@ -93,7 +95,7 @@ class TagGroup:
     RE_ISO8601_DATE = r'^\d{2}(\d{2}|XX)(-\d{2}-\d{2})?$'  # Note: Includes non-specific century dates like '18XX'
     RE_GERMAN_DATE = r'^(?P<dd>\d{2})\.(?P<mm>\d{2})\.(?P<yyyy>\d{4})$'
 
-    def fix_date(self):
+    def fix_date(self) -> TagGroup:
 
         for e in self.group:
             if e.attrib.get('encoding') == 'w3cdtf':
@@ -103,6 +105,9 @@ class TagGroup:
 
         new_group = []
         for e in self.group:
+            if e.text is None:
+                warnings.warn('Empty date')
+                continue
             if e.attrib.get('encoding') == 'iso8601' and re.match(self.RE_ISO8601_DATE, e.text):
                 new_group.append(e)
             elif re.match(self.RE_ISO8601_DATE, e.text):
@@ -131,7 +136,7 @@ class TagGroup:
 
         return self
 
-    def fix_event_type(self):
+    def fix_event_type(self) -> TagGroup:
         # According to MODS-AP 2.3.1, every originInfo should have its eventType set.
         # Fix this for special cases.
 
@@ -161,7 +166,7 @@ class TagGroup:
                     pass
         return self
 
-    def fix_script_term(self):
+    def fix_script_term(self) -> TagGroup:
         for e in self.group:
             # MODS-AP 2.3.1 is not clear about this, but it looks like that this should be lower case.
             if e.attrib['authority'] == 'ISO15924':
@@ -169,7 +174,7 @@ class TagGroup:
                 warnings.warn('Changed scriptTerm authority to lower case')
         return self
 
-    def merge_sub_tags_to_set(self):
+    def merge_sub_tags_to_set(self) -> dict:
         from .mods4pandas import mods_to_dict
         value = {}
 
@@ -189,7 +194,7 @@ class TagGroup:
             value[sub_tag] = s
         return value
 
-    def attributes(self):
+    def attributes(self) -> dict[str, str]:
         """
         Return a merged dict of all attributes of the tag group.
 
@@ -204,7 +209,7 @@ class TagGroup:
                 attrib[a_localname] = v
         return attrib
 
-    def subelement_counts(self):
+    def subelement_counts(self) -> dict[str, int]:
         counts = {}
         for e in self.group:
             for x in e.iter():
@@ -213,7 +218,7 @@ class TagGroup:
                 counts[key] = counts.get(key, 0) + 1
         return counts
 
-    def xpath_statistics(self, xpath_expr, namespaces):
+    def xpath_statistics(self, xpath_expr, namespaces) -> dict[str, float]:
         """
         Extract values and calculate statistics
 
@@ -235,7 +240,7 @@ class TagGroup:
             statistics[f'{xpath_expr}-max'] = np.max(values)
         return statistics
 
-    def xpath_count(self, xpath_expr, namespaces):
+    def xpath_count(self, xpath_expr, namespaces) -> dict[str, int]:
         """
         Count all elements matching xpath_expr
         """
@@ -279,7 +284,7 @@ def _to_dict(root, raise_errors):
         raise ValueError(f"Unknown namespace {root_name.namespace}")
 
 
-def flatten(d: MutableMapping, parent='', separator='_'):
+def flatten(d: MutableMapping, parent='', separator='_') -> dict:
     """
     Flatten the given nested dict.
 
@@ -301,13 +306,13 @@ def flatten(d: MutableMapping, parent='', separator='_'):
     return dict(items)
 
 
-def valid_column_key(k):
-    if re.match("^[a-zA-Z0-9 _@/:\[\]-]+$", k):
+def valid_column_key(k) -> bool:
+    if re.match(r'^[a-zA-Z0-9 _@/:\[\]-]+$', k):
         return True
     else:
         return False
 
-def column_names_csv(columns):
+def column_names_csv(columns) -> str:
     """
     Format Column names (identifiers) as a comma-separated list.
 
diff --git a/src/mods4pandas/mods4pandas.py b/src/mods4pandas/mods4pandas.py
index 2da7c80..ea6a49f 100755
--- a/src/mods4pandas/mods4pandas.py
+++ b/src/mods4pandas/mods4pandas.py
@@ -376,7 +376,7 @@ def pages_to_dict(mets, raise_errors=True) -> List[Dict]:
 @click.option('--output', '-o', 'output_file', type=click.Path(), help='Output Parquet file',
               default='mods_info_df.parquet', show_default=True)
 @click.option('--output-page-info', type=click.Path(), help='Output page info Parquet file')
-def process(mets_files: List[str], output_file: str, output_page_info: str):
+def process(mets_files: list[str], output_file: str, output_page_info: str):
     """
     A tool to convert the MODS metadata in INPUT to a pandas DataFrame.
 
@@ -389,7 +389,7 @@ def process(mets_files: List[str], output_file: str, output_page_info: str):
     """
 
     # Extend file list if directories are given
-    mets_files_real = []
+    mets_files_real: list[str] = []
     for m in mets_files:
         if os.path.isdir(m):
             logger.info('Scanning directory {}'.format(m))