diff --git a/src/sos_notebook/step_executor.py b/src/sos_notebook/step_executor.py
index ed1c801..7f1efa4 100755
--- a/src/sos_notebook/step_executor.py
+++ b/src/sos_notebook/step_executor.py
@@ -21,6 +21,9 @@ def __init__(self, step, mode='interactive'):
     def init_input_output_vars(self):
         # we keep these variables (which can be result of stepping through previous statements)
         # if no input and/or output statement is defined
+        for key in ('step_input', '_depends', 'step_output', '_output', 'step_depends', '_depends'):
+            if key not in env.sos_dict:
+                env.sos_dict.set(key, sos_targets([]))
         if any(x[0] == ':' and x[1] == 'input' for x in self.step.statements):
             env.sos_dict.set('step_input', sos_targets([]))
             env.sos_dict.set('_input', sos_targets([]))