diff --git a/multinut/env.py b/multinut/env.py
index 09fd878..46dc3d7 100644
--- a/multinut/env.py
+++ b/multinut/env.py
@@ -6,6 +6,10 @@ import json
 
 NO_DEFAULT = object()  # Used to indicate no default value was provided
 
+class EnviromentKeyMissing(Exception):
+    def __init__(self, key: str):
+        super().__init__(f"Environment variable '{key}' not found.")
+
 class Modes(Enum):
     """
         Enum for different environment modes.
@@ -107,8 +111,8 @@ class Environment:
             If a cast function is provided, it applies the cast to the value before returning.
         """
         
-        if default is NO_DEFAULT:
-            return cast(self.__values.get(key))
+        if default is NO_DEFAULT and key not in self.__values:
+            raise EnviromentKeyMissing(key)
         
         return cast(self.__values.get(key, default))