diff --git a/neurax/stimulus.py b/neurax/stimulus.py
index ea0191ad2..42f44b7fd 100644
--- a/neurax/stimulus.py
+++ b/neurax/stimulus.py
@@ -8,7 +8,7 @@
 
 
 def step_current(
-    i_delay: float, i_dur: float, i_amp: float, time_vec: jnp.asarray, i_offset=0.0
+    i_delay: float, i_dur: float, i_amp: float, time_vec: jnp.asarray, i_offset: float = 0.0
 ):
     """
     Return step current in unit nA.