diff --git a/pytest_steps/steps_generator.py b/pytest_steps/steps_generator.py
index 4382142..6777f3b 100644
--- a/pytest_steps/steps_generator.py
+++ b/pytest_steps/steps_generator.py
@@ -310,7 +310,8 @@ def execute(self, step_name, *args, **kwargs):
             elif isinstance(res, optional_step):
                 # optional step: check if the execution went well
                 if res.exec_result is None:
-                    raise ValueError("Internal error: this should not happen")
+                    raise ValueError("Internal error: this should not happen."
+                                     "Did you ``yield step_b`` inside the context manager instead of after it?")
 
                 elif isinstance(res.exec_result, OptionalStepException):
                     # An exception happened in the optional step. We can now raise it safely