diff --git a/src/bibapi/schemas/api_types.py b/src/bibapi/schemas/api_types.py
new file mode 100644
index 0000000..5773268
--- /dev/null
+++ b/src/bibapi/schemas/api_types.py
@@ -0,0 +1,81 @@
+from enum import Enum
+
+
+class PicaSchema(Enum):
+    TITLE = "pica.tit"
+    CALLSIGN = "pica.abr"
+    ALL = "pica.all"
+    DATE_FIRST_CREATION = "pica.ser"
+    DATE_LAST_MODIFIED = "pica.aed"
+    ISBN = "pica.isb"
+    ISSN = "pica.isn"
+    ISMN = "pica.ism"
+    PPN = "pica.ppn"
+    AUTHOR = "pica.per"
+    YEAR = "pica.jhr"
+    AUTHOR_SCHEMA = "NoSpaceAfterComma"
+    ENCLOSE_TITLE_IN_QUOTES = False
+
+
+class ALMASchema(Enum):
+    TITLE = "alma.title"
+    AUTHOR = "alma.author"
+    ENCLOSE_TITLE_IN_QUOTES = True
+    AUTHOR_SCHEMA = "NoSpaceAfterComma"
+    YEAR = "date_of_publication"
+
+
+class DublinCoreSchema(Enum):
+    TITLE = "dc.title"
+    AUTHOR = "dc.creator"
+    AUTHOR_SCHEMA = "SpaceAfterComma"
+    ENCLOSE_TITLE_IN_QUOTES = False
+    YEAR = "dc.date"
+
+
+class CQLSchema(Enum):
+    pass
+
+
+class SWBSchema(Enum):
+    URL = "https://sru.k10plus.de/opac-de-627!rec=1?version=1.1&operation=searchRetrieve&query={}&maximumRecords=100&recordSchema=marcxml"
+    ARGSCHEMA = PicaSchema
+    NAME = "SWB"
+    LIBRARY_NAME_LOCATION_FIELD = "924$b"
+
+
+class DNBSchema(Enum):
+    URL = "https://services.dnb.de/sru/dnb?version=1.1&operation=searchRetrieve&query={}&maximumRecords=100&recordSchema=MARC21-xml"
+    ARGSCHEMA = DublinCoreSchema
+    NAME = "DNB"
+
+
+class KOBVSchema(Enum):
+    URL = "https://sru.kobv.de/k2?version=1.1&operation=searchRetrieve&query={}&startRecord=1&maximumRecords=100&recordSchema=marcxml"
+    ARGSCHEMA = DublinCoreSchema
+    NAME = "KOBV"
+    LIBRARY_NAME_LOCATION_FIELD = "924$b"
+
+
+class HebisSchema(Enum):
+    URL = "http://sru.hebis.de/sru/DB=2.1?query={}&version=1.1&operation=searchRetrieve&stylesheet=http%3A%2F%2Fsru.hebis.de%2Fsru%2F%3Fxsl%3DsearchRetrieveResponse&recordSchema=marc21&maximumRecords=100&startRecord=1&recordPacking=xml&sortKeys=LST_Y%2Cpica%2C0%2C%2C"
+    ARGSCHEMA = PicaSchema
+    NOTSUPPORTEDARGS = ["YEAR"]
+    NAME = "HEBIS"
+    REPLACE = {" ": "+", "&": "%26", "=": "+%3D+"}
+    LIBRARY_NAME_LOCATION_FIELD = "924$b"
+
+
+class OEVKSchema(Enum):
+    URL = "https://sru.k10plus.de/opac-de-627-2?version=1.1&operation=searchRetrieve&query={}&maximumRecords=100&recordSchema=marcxml"
+    ARGSCHEMA = PicaSchema
+    NAME = "OEVK"
+    LIBRARY_NAME_LOCATION_FIELD = "924$b"
+
+
+class HBZSchema(Enum):
+    URL = "https://eu04.alma.exlibrisgroup.com/view/sru/49HBZ_NETWORK?version=1.2&operation=searchRetrieve&recordSchema=marcxml&query={}&maximumRecords=100&recordSchema=marcxml"
+    ARGSCHEMA = ALMASchema
+    NAME = "HBZ"
+    LIBRARY_NAME_LOCATION_FIELD = "852$a"
+    NOTSUPPORTEDARGS = ["PPN"]
diff --git a/src/bibapi/schemas/bookdata.py b/src/bibapi/schemas/bookdata.py
index c376bc2..f2d4ba9 100644
--- a/src/bibapi/schemas/bookdata.py
+++ b/src/bibapi/schemas/bookdata.py
@@ -12,7 +12,7 @@ class BookData:
     signature: str | None = None
     edition: str | None = None
     link: str | None = None
-    isbn: Union[str, list[str], None] = field(default_factory=list)
+    isbn: Union[str, list[str], None] = field(default_factory=list[str])
     author: str | None = None
     language: Union[str, list[str], None] = field(default_factory=list)
     publisher: str | None = None