diff --git a/src/mwfin/exceptions.py b/src/mwfin/exceptions.py
new file mode 100644
index 0000000..04572f4
--- /dev/null
+++ b/src/mwfin/exceptions.py
@@ -0,0 +1,10 @@
+class WrongAssumptions(Exception):
+    pass
+
+
+class UnexpectedMarkup(WrongAssumptions):
+    pass
+
+
+class UnknownFinancialStatementItem(WrongAssumptions):
+    pass
diff --git a/tests/test_functions.py b/tests/test_functions.py
index 1ba8200..b1ec9fc 100644
--- a/tests/test_functions.py
+++ b/tests/test_functions.py
@@ -7,6 +7,7 @@ from bs4 import BeautifulSoup
 
 from mwfin import functions
 from mwfin.constants import HTML_PARSER, BASE_URL, FIN_STMT_URL_SUFFIX, IS, BS, CF, END_DATE
+from mwfin.exceptions import UnknownFinancialStatementItem
 
 
 THIS_DIR = Path(__file__).parent
@@ -62,7 +63,8 @@ class FunctionsTestCase(IsolatedAsyncioTestCase):
         test_soup = BeautifulSoup('<tr><td><div> Cash & Short Term Investments Growth </div></td></tr>', HTML_PARSER)
         self.assertFalse(functions.is_relevant_table_row(test_soup.tr))
         test_soup = BeautifulSoup('<tr><td><div> baz </div></td></tr>', HTML_PARSER)
-        self.assertFalse(functions.is_relevant_table_row(test_soup.tr))
+        with self.assertRaises(UnknownFinancialStatementItem):
+            functions.is_relevant_table_row(test_soup.tr)
 
     @patch.object(functions, 'is_relevant_table_row')
     def test_find_relevant_table_rows(self, mock_is_relevant_table_row):
@@ -72,6 +74,11 @@ class FunctionsTestCase(IsolatedAsyncioTestCase):
         self.assertListEqual(expected_output, output)
         mock_is_relevant_table_row.assert_has_calls([call(expected_output[0]), call(expected_output[1])])
 
+        mock_is_relevant_table_row.side_effect = UnknownFinancialStatementItem()
+        expected_output = self.test_soup.find_all('thistagdoesntexist')
+        output = functions.find_relevant_table_rows(self.test_soup)
+        self.assertListEqual(expected_output, output)
+
     def test_extract_row_data(self):
         test_row = self.test_soup.find('div', attrs={'class': 'financials'}).tbody.tr
         expected_output = ('Cash & Short Term Investments', (11000000, -22000000))