diff --git a/doc/source/tutorial/magnetic_field.rst b/doc/source/tutorial/magnetic_field.rst
index 66bfb80575db9e0ad9c9c4ef18196acf9a1dec9d..add388bfccdeb5505343a3c0785a1f74563e9056 100644
--- a/doc/source/tutorial/magnetic_field.rst
+++ b/doc/source/tutorial/magnetic_field.rst
@@ -37,7 +37,7 @@ operators with the substitution
 .. math::
 
     k_x = \frac{1}{\sqrt{2} l_B} (a + a^\dagger), \quad \quad
-    k_y = \frac{1}{\sqrt{2} l_B} (a - a^\dagger),
+    k_y = \frac{i}{\sqrt{2} l_B} (a - a^\dagger),
 
 with the magnetic length :math:`l_B = \sqrt{\hbar/eB}`. The ladder operators obey the
 commutation relation