Background: Every time a customer buy something from us we will ask if the customer wants to become a member. I'm trying to build a Logistic Regression model to forecast how likely a customer will become a member based on some attributes and I'm facing a problem about how to deal with missing data properly in this case.
Question: I found there is a significant correlation between "Time Waiting in Line" and "Become a Member". However, "Time Waiting in Line" only make sense when a customer walks in our store but doesn't make sense when a customer buys things online or using phone calls. Therefore, I don't think it's OK to fill in the missing "Time Waiting in Line" using median or average... What should I do here?