Unions are more popular than they’ve been in almost two decades, thanks to successful organizing efforts in new sectors like digital media and fast food, revitalized labor action in more traditional industries like education and manufacturing, and the threat of a general strike helping end a painful government shutdown. The labor movement is behind the greatest advances for working people in this country’s history — from the eight-hour workday to the minimum wage and the end of child labor — and built the United States middle class as we know it.

However, due to a flaw in a major labor law passed back during the New Deal era, not everyone is able to join a union.

Following the passage of the 1926 Railway Labor Act, which oversaw labor relations in the railroad and airline industries, President Franklin Delano Roosevelt signed the National Labor Relations Act (NLRA) of 1935, which has remained the bedrock of federal labor law in the U.S. The NLRA guarantees the majority of private sector workers the right to organize and join unions, engage in collective bargaining with their employers, and take collective action, like striking, when deemed necessary. Under this law, no employer can fire or threaten to fire a worker for organizing or joining a union or being pro-union, something that many bosses neglect to tell said workers during efforts at union-busting.

Because of the NLRA, thousands of workers across the country have been able to organize and bargain for better wages and working conditions, but, because of the way the law was written, thousands of others have been left without a legal option to do either.

Government employees, agricultural laborers, independent contractors, and supervisors (with limited exceptions), as well as domestic workers and those covered by the Railway Labor Act are excluded from the NLRA. Some of these workers are also not covered by the Occupational Safety and Health Act (which oversees workplace safety) or the Fair Labor Standards Act (which regulates wages and hours); in addition, those classified as independent contractors are responsible for paying their own federal and state income taxes (which, as anyone who has to do this can tell you, adds up quickly). These excluded worker categories also often represent some of the most vulnerable people. For example, an overwhelming number of agricultural workers are subject to low wages and dangerous working conditions, and while they do have some specific protections, those efforts still fall short.

It’s also important to note the racist history behind the exclusion of certain worker categories. Today, most U.S. farm workers come from Latin American countries like Mexico and Guatemala, and about half of those workers are undocumented. But at the time of the NLRA’s signing, the majority of agricultural workers were black, and some scholars believe it’s no coincidence that these workers were excluded from the NLRA.

As Juan F. Perea notes in his paper “The Echoes of Slavery: Recognizing the Racist Origins of the Agricultural and Domestic Worker Exclusion from the National Labor Relations Act”: “Southern congressmen wanted to exclude black employees from the New Deal to preserve the quasi-plantation style of agriculture that pervaded the still-segregated Jim Crow South.”

This imperfect law still governs the lives of working people today, and the way an employer uses it to classify the people who work for them has a huge impact on an employee’s ability to protect and advocate for themselves. Worker misclassification under the NLRA is the source of many of the ongoing labor disputes in multiple industries, from tech to digital media.

That means that there are millions of workers who are left unprotected by the country’s most important labor law, and who can’t join most unions even if they wanted to. “There isn’t a clear-cut way for workers to determine if they are misclassified on their own,” Amity Paye, a representative from Service Employees International Union Local 32BJ, tells Teen Vogue.