diff --git a/packages/backend-core/src/middleware/joi-validator.ts b/packages/backend-core/src/middleware/joi-validator.ts
index 7575e7e778..a85c0e7108 100644
--- a/packages/backend-core/src/middleware/joi-validator.ts
+++ b/packages/backend-core/src/middleware/joi-validator.ts
@@ -6,7 +6,7 @@ function validate(
   property: string,
   opts?: { errorPrefix?: string; allowUnknown?: boolean }
 ) {
-  const errorPrefix = opts?.errorPrefix || `Invalid ${property}`
+  const errorPrefix = opts?.errorPrefix ?? `Invalid ${property}`
   // Return a Koa middleware function
   return (ctx: Ctx, next: any) => {
     if (!schema) {