Knowledge

ERROR_IS_SUBSTED - 135 (0x87)

An attempt was made to use a JOIN or SUBST command on a drive that has already been substituted.

Updated: Feb 21, 2026

Introduction

This article provides a detailed explanation of the ERROR_IS_SUBSTED error, its technical background, common causes, and how to diagnose and resolve it.

Technical Background

The ERROR_IS_SUBSTED error is a specific Windows API error that occurs when an attempt is made to use the JOIN or SUBST command on a drive that has already been substituted. This error typically manifests in scenarios where a user or application tries to create a symbolic link (using JOIN) or substitute a drive letter (using SUBST) for a drive that is already mapped.

Error Details

  • Error Name: ERROR_IS_SUBSTED
  • Numeric Code: 135
  • Hex Code: 0x87
  • Short Description: An attempt was made to use a JOIN or SUBST command on a drive that has already been substituted.

Common Causes

The error ERROR_IS_SUBSTED is logically implied by the following scenarios:

  • Attempting to create a symbolic link (JOIN) for a drive that is already mapped.
  • Trying to substitute a drive letter (SUBST) for a drive that is already assigned.

Real-World Context

This error can occur in various contexts, such as when a user or an application attempts to map a network drive using the SUBST command on a drive that has already been mapped. It also applies to scenarios where symbolic links are created for drives that are already present in the system.

Is This Error Critical?

The criticality of this error depends on the context and the specific operation being performed. While it does not typically impact system stability or data integrity, it can prevent certain operations from completing successfully.

How to Diagnose

To diagnose ERROR_IS_SUBSTED, follow these steps:

  1. Review Operation Context: Ensure that no other processes are using the drive in question.
  2. Validate Parameters: Check if the command is being used correctly, ensuring that the drive letter and path are valid.
  3. Confirm Object Types: Verify that the operation is not being performed on a drive that is already mapped or substituted.
  4. Verify Input Data: Ensure that there are no conflicting operations or mappings for the same drive.

How to Resolve

To resolve ERROR_IS_SUBSTED, consider the following actions:

  • Correct Parameter Usage: Ensure that the correct parameters are used in the command, and that the drive is not already mapped.
  • Adjust Operation Context: If necessary, unmap or remove any existing mappings before performing the operation again.

Developer Notes

Developers should be cautious when implementing operations that involve drive substitution or symbolic links to avoid encountering this error. Proper validation of parameters and context can prevent such issues from arising.

Related Errors

  • ERROR_ALREADY_ASSIGNED (1245): This error occurs when an attempt is made to assign a device name that is already in use.
  • ERROR_INVALID_DRIVE (16387): This error indicates that the specified drive letter or path is invalid.

FAQ

Q: What does ERROR_IS_SUBSTED mean?

A: It means an attempt was made to use a JOIN or SUBST command on a drive that has already been substituted.

Q: How can I prevent this error from occurring?

A: Ensure that the drive is not already mapped before attempting to substitute it using SUBST, and verify that no symbolic links are present for the same drive when using JOIN.

Summary

The ERROR_IS_SUBSTED error is a specific Windows API error indicating an attempt to use a JOIN or SUBST command on a drive that has already been substituted. Understanding its context, causes, and resolution can help in effectively managing operations involving drive substitution and symbolic links.