Twitter (NYSE:TWTR) saw its stock surge tempered by downgrades from two Wall Street analysts Monday.

Analysts at Wells Fargo Securities and SunTrust Robinson Humphry slashed their investment ratings on the ubiquitous microblog after its shares jumped more than 40 percent this month, The Wall Street Journal reported. While investor sentiment has turned decidedly more optimistic recently, analysts are saying Twitter’s fundamentals no longer support the current valuation.

“We believe investors underestimate some challenges facing the company and advertisers seeking to employ the platform,” Peter Stabler, senior analyst at Wells Fargo Securities, wrote to clients. He downgraded his rating to underperform from market perform, while maintaining a $36-$39 valuation range on the stock.

SunTrust’s Robert Peck, who downgraded the stock to neutral from buy on valuation concerns, said it had “become stretched” since it was trading about 20 percent above his $50 price target, the Journal reported, citing Fly on the Wall.

Twitter shares closed Monday at $56.61, down 4.05 percent, but gained 4 cents in after-hours trading.