Wellens told BBC Radio Leicester: "Everyone should be frustrated.
"We put in a very good performance in terms of playing but you can't defend like that - the way they scored the goals was far too easy."
The Royals cruised into a 2-0 lead and added a third after the break, with Leicester's sole reply from Andy King coming in the latter stages.
"We had to work really hard to get some shots," Wellens continued.
"They had two or three cleared off the line where their defenders are throwing their bodies on the line and clearing it and we've let in two simple goals in the first half.
"The last two away games we have
conceded three at Middlesbrough
and three against Reading and we've dominated both games, so I'd hate to see us if we were under pressure. We can't concede goals like that."
The Foxes face fellow play-off hopefuls Nottingham Forest on Friday with both sides knowing a win is needed to keep alive their promotion bid.
Wellens added: "We've got to win and we've got to defend better."
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