Amazon’s shares fell nearly 6% on Monday after an analyst dropped his rating for the e-commerce giant because of concerns about slower growth.

James Cakmak, with brokerage firm Monness Crespi Hardt, dropped Amazon’s rating from “buy” to “neutral,” citing concerns about the rising costs for its cloud computing business and for developing original shows to stream on its Prime subscription video service. He worried that the company will be unable to maintain the momentum of last year, when its shares rose 118%.

“We believe even winners need to take breathers sometimes as estimates recalibrate,” Cakmak wrote in his note (as cited in Barron’s). “With a multi-year horizon, we are still very much positive on the name. Over the near-term, however, we believe there may prove a better entry point to initiate or add to positions. Our timing may be premature in light of likely solid performance stemming from the December quarter, but the diminishing upside we see to estimates which already reflect an optimistic outlook give us pause for now.”

WATCH: For more on Amazon’s cloud business, watch this Fortune video:

The skepticism comes despite hints from Amazon that it may have had a solid fourth quarter during the all-important holiday season. Although famously tight-lipped about its business details, the company did recently reveal that 3 million people signed up for its Prime service in the third week of December—an achievement the company described as “milestone growth.” Amazon also said that the sales of its devices more than doubled last year’s record-setting number.

Amazon’s shares closed down $38.90 at $636.99 in regular trading Monday, as the broader market also declined.

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