Movus/iStock Editorial via Getty ImagesCoinbase Global (COIN) stock falls 2.9% after Keefe, Bruyette & Woods analyst Kyle Voight points to a couple unknowns surrounding the crypto app's recently introduced 4% crypto savings account.To be sure, bitcoin itself is weaker today, down 2.6% over the past 24 hours to ~$33.2K."The pre-enrollment announcement is relatively in line with the timing for the current revenue stamp in our published model," Voight said in a note to clients.Questions remain about the company's "principal guarantee" and net interest spread guidance, he said.COIN offers its clients a "principal guarantee" on USD Coin holdings, although it's uncertain if this is through third party insurance provided to COIN, or if it's a guarantee made by Coinbase itself;And it has not yet given guidance on its net interest spread. Voight notes that 25% commission on gross staking revenues, which means a 4% client annual percentage yield on USDC holdings is equivalent to 1%-1.5% net