diff --git a/tests/test_cocos.py b/tests/test_cocos.py
index 71b67ae..a7357a2 100644
--- a/tests/test_cocos.py
+++ b/tests/test_cocos.py
@@ -4,7 +4,7 @@
 import numpy as np
 import pytest
 
-from pyloidal.cocos import Transform, identify_cocos
+from pyloidal.cocos import Sigma, Transform, identify_cocos
 
 ALL_COCOS = list(range(1, 9)) + list(range(11, 19))
 
@@ -75,3 +75,15 @@ def test_cocos_transform():
         for cocos_add in (cocos + x * 10 for x in range(2)):
             for key in ("b_toroidal", "toroidal", "poloidal", "q"):
                 assert getattr(Transform(cocos_add, cocos_add), key) == 1
+
+
+def test_sigma_bad_inputs():
+    """Test that Sigma raises an excpetion when given an inputs not in ``(-1, 1)``"""
+    with pytest.raises(ValueError, match="B_poloidal"):
+        Sigma(B_poloidal=2, r_phi_z=1, r_theta_phi=-1)
+
+    with pytest.raises(ValueError, match="r_phi_z"):
+        Sigma(B_poloidal=-1, r_phi_z=0, r_theta_phi=-1)
+
+    with pytest.raises(ValueError, match="r_theta_phi"):
+        Sigma(B_poloidal=-1, r_phi_z=1, r_theta_phi=-5)