diff --git a/src/tests/test_math.py b/src/tests/test_math.py
deleted file mode 100644
index 96197fb..0000000
--- a/src/tests/test_math.py
+++ /dev/null
@@ -1,9 +0,0 @@
-import pytest
-from datetime import datetime
-from xirr.math import xirr
-
-
-@pytest.mark.parametrize("valuesPerDateString,expected", [({'2019-12-31': -80005.8, '2020-03-12': 65209.6}, -0.6453638827)])
-def test_xirr(valuesPerDateString, expected):
-    valuesPerDate = {datetime.fromisoformat(k).date: v for k, v in valuesPerDateString.items()}
-    assert xirr(valuesPerDate) == expected
diff --git a/src/xirr/math.py b/src/xirr/math.py
index 58754d6..a190dce 100644
--- a/src/xirr/math.py
+++ b/src/xirr/math.py
@@ -37,7 +37,7 @@ def xirr(valuesPerDate):
     >>> xirr(valuesPerDate)
     -0.6454
     '''
-    if not valuesPerDate:
+    if not valuesPerDate or len(valuesPerDate) < 2:
         return None
 
     result = None
diff --git a/tests/test_math.py b/tests/test_math.py
index c03b71e..9a17155 100644
--- a/tests/test_math.py
+++ b/tests/test_math.py
@@ -10,6 +10,7 @@
     ({'2020-03-12': 65209.6, '2019-12-31': -80005.8}, -0.6454),
     ({'2019-12-31': -100082.76, '2020-03-05': 82671.24}, -0.6581),
     ({}, None),
+    ({'2019-12-31': -100082.76}, None),
     ({'2019-12-31': -0.00001, '2020-03-05': 0.00001}, 0.0),
     ({'2019-12-31': -100, '2020-03-05': 100}, 0.0),
     ({'2019-12-31': -100, '2020-03-05': 1000}, 412461.6383),