Ohio-based electric vehicle startup Workhorse is suspending deliveries of its C-1000 electric delivery vans and recalling 41 vehicles that have already been sent to customers, telling regulators the vans need “additional testing and modifications” in order to comply with federal safety standards.

It was the latest setback for the company, which recently abandoned its effort to overturn the US Postal Service’s decision to let a different company build the next-generation mail truck. That contract was awarded to defense manufacturer Oshkosh earlier this year.

The C-1000 is meant to be Workhorse’s flagship vehicle, with 1,000 cubic feet of cargo space. The company said it was continuing to tinker with the design, even as it finally got the vehicle into production this past quarter following years of struggles.

But apparently that wasn’t enough to stave off a recall. In a filing with the Securities and Exchange Commission, Workhorse said it identified “a number of enhancements in the production process and design of the C-1000 to address customer feedback, primarily related to vehicle dynamics to increase the vehicles’ payload capacity.”

“We have identified a number of opportunities to improve our C-1000 series vehicles and are committed to getting these previously delivered vehicles back on the road,” Workhorse CEO Rick Dauch said in a statement. “Importantly, we remain on track to communicate our new, long-term strategic roadmap to enhance our trucks and operational capabilities on our third quarter earnings call. We continue to be confident in our ability to be a leading manufacturer of last-mile delivery vehicles over the long term.”

The company says it intends to complete its testing of the C-1000 in the fourth quarter of this year and will provide an update to investors during the third quarter earnings report.