diff --git a/sigkernel/static_kernels.py b/sigkernel/static_kernels.py
index d503651..30c1cc4 100644
--- a/sigkernel/static_kernels.py
+++ b/sigkernel/static_kernels.py
@@ -116,7 +116,7 @@ def Gram_matrix(self, X, Y):
 class RBF_SQR_Kernel():
     """RBF SQR kernel k: H x H -> R"""
 
-    def __init__(self, sigma1, sigma2):
+    def __init__(self, sigma_1, sigma_2):
         self.rbf1 = RBFKernel(sigma_1)
         self.rbf2 = RBFKernel(sigma_2)