diff --git a/packages/playwright/src/common/config.ts b/packages/playwright/src/common/config.ts
index 015dbc1e71..ac7b313820 100644
--- a/packages/playwright/src/common/config.ts
+++ b/packages/playwright/src/common/config.ts
@@ -107,7 +107,7 @@ export class FullConfigInternal {
         const cpus = os.cpus().length;
         this.config.workers = Math.max(1, Math.floor(cpus * (parseInt(workers, 10) / 100)));
       } else {
-        this.config.workers = parseInt(workers, 10);
+        this.config.workers = parseWorkers(workers);
       }
     } else {
       this.config.workers = workers;
@@ -223,6 +223,14 @@ function resolveReporters(reporters: Config['reporter'], rootDir: string): Repor
   });
 }
 
+function parseWorkers(workers: string) {
+  const parsedWorkers = parseInt(workers, 10);
+  if (isNaN(parsedWorkers))
+    throw new Error(`Workers ${workers} must be a number or percentage.`);
+
+  return parsedWorkers;
+}
+
 function resolveProjectDependencies(projects: FullProjectInternal[]) {
   const teardownSet = new Set<FullProjectInternal>();
   for (const project of projects) {