A Wall Street analyst who had been bullish on Workhorse Group (WKHS -14.93%) gave the electric-van company's shares a downgrade on Thursday.

In a note on Thursday morning, Colliers analyst Michael Shlisky cut his rating on the electric delivery-van maker to neutral, from buy, and removed his earlier $11 price target. 

Noting that Workhorse's share price has increased by over 500% in the last six weeks, Shlisky wrote that while he's "thrilled" that investors have recognized the potential of the company's battery-electric C-Series vans and other products, he's concerned that investors have yet to fully price in some of the uncertainties in Workhorse's story. 

The company's C-Series is an electric van optimized for so-called "last mile" deliveries, meaning the final deliveries of parcels to recipients. Developed with input from United Parcel Service (UPS -1.22%), which has ordered over 1,000 of the vehicles, the C-Series received final U.S. government approval last month. 

A red Workhorse C-650, an electric package-delivery van.

Workhorse's electric C-Series vans are optimized for package deliveries. Image source: Workhorse Group.

With that approval in hand, Workhorse is expected to bid on an upcoming United States Postal Service contract for zero-emission mail delivery vehicles. A win would be a major milestone for the company, but stiff competition is expected. 

Workhorse expects to deliver 300 to 400 of the C-Series vans by the end of 2020. 

Shlisky's note encouraged investors to "take a moment to recharge" while awaiting the company's upcoming second-quarter earnings report, expected next month.

Workhorse's shares were down about 3% in early trading on Thursday.