The fear is that Trump, once in office, will quickly act on his promise to upend U.S. immigration laws. Any changes could create “unpredictability” at ports of entry and maroon workers outside the country or cause significant delays in their return.

The Amazon employee, who requested anonymity to avoid jeopardizing his H-1B status within the company’s cloud computing division, took the warning to stay in the U.S. seriously. He jettisoned plans for a late-January trip to Cancun, to trade Seattle gloom for some sunshine, in favor of a domestic destination. “We decided on Hawaii,” he said.

Amazon is among many companies that have warned foreign workers to return from any international travel before Trump takes his oath of office. For some workers, that means delaying or cancelling vacations and trips to see family overseas, leaving many anxious and uncertain about their futures.

Shortly after taking office in his first term, Trump issued an executive order blocking the immigration of people born in seven predominantly Muslim countries. The decision created immediate chaos at airports and led to visas for up to 60,000 people being provisionally revoked.

This time around, immigration is again in Trump’s crosshairs and those of his supporters. For example, one of the temporary worker visa programs, the H-1B, designed for skilled workers, came under fire in recent weeks as Trump supporters debated whether tech companies exploit the program to prioritize lower-paid foreign employees over hiring Americans.

Although Trump recently voiced support for such programs, he has also promised mass deportations. Well aware of Trump’s unpredictability, many companies are deciding to play it safe.

A current employee of U.S. Bank who is from India on an H-1B visa, also told Fortune he had received a similar letter as Amazon’s from an immigration law firm engaged by the company. The letter, he said, included advice to return to the U.S. before January 20 and recommended reaching out to the law firm if immigration officers called him.

As with all anonymous sources in this article, Fortune knows the worker’s identity.

Another tech employee at a different company showed Fortune a similar memo sent last month to one of his friends who also works in tech. The memo, which had the company name redacted, “highly recommended” that employees who have international travel planned return to the U.S. “before the new U.S. government’s administration takes office, “out of an abundance of caution” except for urgent situations or emergencies related to health or business matters.

The letter, provided under the condition of anonymity because it was a confidential document, also recommended that employees delay unnecessary travel during the Trump administration’s first few months. The letter said “we do not want to cause any undue alarm,” but considering the possibility of a travel ban, “it would be prudent to allow for a bit of time for the new administration to take office and to let the dust settle.” 

Other foreign tech workers who went to U.S. universities have received warnings from their alma maters. A letter from the University of Massachusetts at Amherst recommended that its international community—including all international students, scholars, and faculty and staff whose immigration status is sponsored by the school—to “strongly consider returning to the United States prior to the presidential inauguration day” if they are traveling internationally.

The university’s letter acknowledged that its warning isn’t based on any current U.S. government policy or recommendation. But it continued by saying that “given a new presidential administration can enact new policies on their first day in office (January 20), and based on previous experience with travel bans that were enacted in the first Trump Administration in 2016, the Office of Global Affairs is making this advisory out of an abundance of caution to hopefully prevent any possible travel disruption to members of our international community.”

Poonam Gupta, a New York-based immigration lawyer who advises Big Tech employees, says the travel advice in the letters obtained by Fortune align with what she has been advising her clients since the presidential election results were announced in November. “We anticipate a few executive orders to be issued on January 20 and while they may not be directed (we hope) towards the work visas, there is always impact across the board,” she wrote in an email.

And it is not just Big Tech companies, but also small employers, that are advising foreign individuals to complete all travels before January 20, said Natalia Polukhtin, an immigration attorney in Scottsdale, Ariz. She’s advised corporate clients about how to warn potentially impacted employees. “We anticipate rapid shifts in U.S. policies that may result in the denial of entry at the border,” Polukhtin told Fortune in an email. “With something similar to the ‘Muslim ban’ enacted through the executive order during the prior Trump’s Administration, it is very feasible that citizens of ‘undesirable’ countries may be locked out of the U.S. even with perfectly valid travel documents.”

The warnings from companies reviewed by Fortune made no distinction between temporary work visas, such as H-1Bs, or green cards, for permanent residents. The email Amazon sent to impacted employees had a subject line saying “Holiday Travel Guidelines for Foreign Nationals” and opened with an explanation saying “you are receiving this message as our team has previously provided you with immigration support.”

The subject appears to be a sensitive one for companies: Amazon did not respond to a request for comment, nor did Google. Meanwhile, Nvidia and Microsoft declined to comment.

Not all companies are sending travel guidance to workers who are foreign nationals. Sourav Bhattacharyya, an Indian national who works for a large Pittsburgh-based technology consulting firm under an H-1B visa, said that he needs to travel to Europe in March as part of a required course to complete his executive MBA. When he asked his company’s immigration legal team about any concerns about the trip, he was told there is no official travel advisory. Still, Bhattacharyya is worried.

“Who knows if President Trump tries to impose a travel ban or ask the Department of Homeland security to increase scrutiny at the POEs [Ports of Entry] across the country? While returning back to the U.S., we might end up facing needless hassle,” he said.

Sathyaraj Radhakrishnan, another Indian national who works in AI and analytics for consulting firm Cognizant on an H-1B visa, also said he had not received a travel advisory from his employer. But he’s taking his precautions anyway.

His father had surgery last week in India, but because of the possibility of Trump’s signing new executive orders on immigration, he said he is “avoiding travel for at least a month.” Although India was not one of the countries affected by the 2017 ban, “I would rather have things settle down before I make concrete plans,” Radhakrishnan added.