I concur with the review from Lochness posted 9 months ago. 
The base of the propane unit has a hole in the base into which the propane tank sits.
For the natural gas model it would have been such a simple and inexpensive modification to replace that base with a solid one. At the very least Napoleon could have provided a plate to close that gap, and prevent squirrels, mice or chipmunks from entering. A disappointing oversight. If this was done as a shortcut to avoid the modest additional engineering and manufacturing expense to make it a "legitimate" natural gas model, that is shameful for a company that promotes itself as the industry leader in high quality and technically advanced bbq's. Napoleon's response to Lochness's review demonstrates how little they value or act on customer input.