diff --git a/src/logic/webrequest.py b/src/logic/webrequest.py
index 8146adf..fa47c86 100644
--- a/src/logic/webrequest.py
+++ b/src/logic/webrequest.py
@@ -88,9 +88,23 @@ class WebRequest:
 
 
 class BibTextTransformer:
-    def __init__(self, mode: str) -> None:
+    """Transforms data from the web into a BibText format.
+        Valid Modes are ARRAY, COinS, BibTeX, RIS, RDS
+    Raises:
+        ValueError: Raised if mode is not in valid_modes
+
+    Returns:
+        None
+    """
+
+    valid_modes = ["ARRAY", "COinS", "BibTeX", "RIS", "RDS"]
+
+    def __init__(self, mode: str = "ARRAY") -> None:
         self.mode = mode
         self.field = None
+        if mode not in self.valid_modes:
+            logger.log_error(f"Mode {mode} not valid")
+            raise ValueError(f"Mode {mode} not valid")
         # print(self.field)
         self.data = None
         # self.bookdata = BookData(**self.data)