President Donald Trump’s proposal for European countries to provide Ukraine with air defenses may run into an issue, as Germany is hesitant to send its Patriot battery to Kiev without a firm plan from Washington to give Berlin priority access to a replacement.

In an interview with Der Spiegel, German Defense Minister Boris Pistorius explained Berlin expects to replace any Patriot missile systems it sends to Ukraine in six to eight months. “Our expectation is that NATO will then approach the US again and make it clear to the manufacturer of the Patriot systems that the countries that are handing over their systems must be supplied with new systems within a few months,” he said.

Pistorius said the pledge must be “watertight,” adding, “no major gaps are created that endanger our security and that of our allies.” He continued, “It’s not just about the current operational capability of our air defense. It’s also about being able to conduct continuous training on Patriot systems, which requires a minimum number of systems.”

The issue with the demand is a shortage of Patriot batteries and interceptors. If Germany is given priority access to the next battery produced, it means other countries will be pushed down the wait list.

Additionally, the US is suffering from a shortage of Patriot interceptors, with the Pentagon estimating that it only has 25% of the missiles it needs to carry out its defense plans.

Ukraine is in desperate need of air defense systems as Russia has stepped up missile and drone attacks. On Thursday, Russia reported striking over 130 targets across Ukraine.