diff --git a/pynam/dielectric/nam_dielectric_coefficient_approximator.py b/pynam/dielectric/nam_dielectric_coefficient_approximator.py
index deedb80..f3a26db 100644
--- a/pynam/dielectric/nam_dielectric_coefficient_approximator.py
+++ b/pynam/dielectric/nam_dielectric_coefficient_approximator.py
@@ -36,9 +36,9 @@ class NamDielectricCoefficients(object):
 		self.d = d
 		self.u_l = np.real((-self.c + 1j * self.d) / (-self.a + 1j * self.b))
 
-	def eps(self):
+	def eps(self, u_c: float):
 
-		def piecewise_eps(u: float, u_c: float):
+		def piecewise_eps(u: float):
 			# todo add check for u_c vs u_l
 			if u < self.u_l:
 				return -self.a + 1j * self.b
diff --git a/tests/dielectric/test_nam_dielectric_coefficient_approximator.py b/tests/dielectric/test_nam_dielectric_coefficient_approximator.py
index 48e6f74..2a604a7 100644
--- a/tests/dielectric/test_nam_dielectric_coefficient_approximator.py
+++ b/tests/dielectric/test_nam_dielectric_coefficient_approximator.py
@@ -15,7 +15,8 @@ import pynam.dielectric.nam_dielectric_coefficient_approximator
 ])
 def test_dedimensionalise_parameters(test_input, expected):
 
-	actual_parameters = pynam.dielectric.nam_dielectric_coefficient_approximator.get_dedimensionalised_parameters(*test_input)
+	actual_parameters = pynam.dielectric.nam_dielectric_coefficient_approximator.get_dedimensionalised_parameters(
+		*test_input)
 
 	np.testing.assert_almost_equal(
 		actual_parameters.xi, expected[0],
@@ -52,7 +53,8 @@ def test_dedimensionalise_parameters(test_input, expected):
 ])
 def test_nam_coefficients(test_input, expected):
 
-	actual_coefficients = pynam.dielectric.nam_dielectric_coefficient_approximator.get_nam_dielectric_coefficients(*test_input)
+	actual_coefficients = pynam.dielectric.nam_dielectric_coefficient_approximator.get_nam_dielectric_coefficients(
+		*test_input)
 
 	np.testing.assert_allclose(
 		actual_coefficients.a, expected[0],
@@ -86,19 +88,19 @@ def test_nam_eps():
 		2e6,
 		0.8e11,
 		1e11,
-		3e8).eps()
+		3e8).eps(u_c)
 
 	np.testing.assert_allclose(
-		eps_to_test(10, u_c), -3.789672906817707e10 + 3.257134605133221e8j,
+		eps_to_test(10), -3.789672906817707e10 + 3.257134605133221e8j,
 		rtol=1e-3, err_msg='below u_l bad'
 	)
 
 	np.testing.assert_allclose(
-		eps_to_test(1e10, u_c), -2.655709887616547e8 + 2.302290450767144e6j,
+		eps_to_test(1e10), -2.655709887616547e8 + 2.302290450767144e6j,
 		rtol=1e-3, err_msg='linear region bad'
 	)
 
 	np.testing.assert_allclose(
-		eps_to_test(1e17, u_c), 1,
+		eps_to_test(1e17), 1,
 		rtol=1e-6, err_msg='above cutoff bad'
 	)