diff --git a/src/classes.py b/src/classes.py
index 8514d92..c0d23bb 100644
--- a/src/classes.py
+++ b/src/classes.py
@@ -83,10 +83,10 @@ class Entrypoint:
     def __init__(self, sample_data):
         try:
             self.extra = sample_data["metadata_decoded"]["extra_fields"]
-            self.linked_items = sample_data["items_links"]
-            self.batch_elabid = self.extra["Substrate batch"]["value"]
-            self.linked_experiments = sample_data["related_experiments_links"]
-            self.linked_experiments_elabid = [ i["entityid"] for i in self.linked_experiments ]
+            self.linked_items = sample_data["items_links"] # dict
+            self.batch_elabid = self.extra["Substrate batch"]["value"] # elabid
+            self.linked_experiments = sample_data["related_experiments_links"] # dict
+            self.linked_experiments_elabid = [ i["entityid"] for i in self.linked_experiments ] # list of elabid
         except KeyError as k:
             # Some keys are not required and can be called through the .get() method - which is permissive and allows null values;
             # Other keys are required so if they can't be called (invalid or null) raise error and stop execution of the program:
diff --git a/src/functions.py b/src/functions.py
index e4133eb..ab48f35 100644
--- a/src/functions.py
+++ b/src/functions.py
@@ -1,3 +1,6 @@
+"""
+Currently unused!
+"""
 import json, requests
 from APIHandler import APIHandler
 
diff --git a/src/main.py b/src/main.py
index c854d7a..8846b38 100644
--- a/src/main.py
+++ b/src/main.py
@@ -44,6 +44,8 @@ if __name__=="__main__":
         elif data.get("category_title") in ["PLD Target", "Substrate"]:
             item = Material(data)
             print(item.get_compound_formula(apikey))
+        else:
+            raise Exception("The selected item or experiment is not in one of the following categories: [Sample, PLD Target, Substrate, PLD Deposition].")
         result = item.__dict__
         result.pop("extra")
         print(result)
\ No newline at end of file