How to fix Error 4185 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4185 – “This action cannot be performed on a system type”

Why This Error Happens

Error 4185 occurs when a user or application attempts to execute an action—such as ALTER, DROP, or modifying properties—on a system-defined data type or system object that SQL Server protects from direct manipulation. This typically happens when scripts or migration tools mistakenly target system types (like sys.types entries) instead of user-defined types (UDTs), or when deployment packages generated for one database are replayed against system catalogs. Corrupted or outdated DACPAC/BACPAC files and third-party ORM tools that auto-generate schema scripts are common triggers for this conflict in SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Object

  • Open SQL Server Management Studio (SSMS) and run the following query to confirm whether the target object is a system type:
  SELECT name, is_user_defined FROM sys.types WHERE name = 'YourTypeName';
  • If is_user_defined = 0, this confirms the object is a protected system type and cannot be altered directly.
  • Check the SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for the exact timestamp and query context tied to Error 4185.

Step 2: Correct the Script or Deployment Package

  • If using SSDT or Visual Studio Database Projects, review the .sql deployment script for any ALTER TYPE or DROP TYPE statements referencing base types (e.g., int, varchar, sysname).
  • Edit the script to target only user-defined types, typically prefixed or stored under a custom schema (e.g., dbo.MyCustomType).
  • If the issue stems from a DACPAC, regenerate it using:
  SqlPackage.exe /Action:Extract /SourceConnectionString:"Server=localhost;Database=YourDB;Integrated Security=true" /TargetFile:"C:\Temp\YourDB.dacpac"

Step 3: Validate Permissions and Ownership

  • Ensure the executing login does not have elevated rights (like sysadmin) that might allow partial execution against system catalogs, masking the real intent of the failing script.
  • Run the following to check type ownership:
  SELECT t.name, s.name AS SchemaOwner 
  FROM sys.types t
  JOIN sys.schemas s ON t.schema_id = s.schema_id
  WHERE t.name = 'YourTypeName';
  • Reassign ownership if the user-defined type was mistakenly created in the sys or dbo system-reserved namespace.

Step 4: Re-run the Operation in Isolation

  • Isolate the failing statement and test it independently in a new query window against a non-production database copy.
  • Use SET NOEXEC ON; before execution to validate syntax without committing changes, then toggle SET NOEXEC OFF; once confirmed safe.

Alternative or Verification Steps

  • Verification: Re-run the original deployment or migration script. If successful, query sys.messages for Error 4185 to confirm no new occurrences are logged in the SQL Server Error Log.
  • Alternative Fix: If the error persists during automated CI/CD deployments, bypass the conflict by explicitly excluding system schema comparisons in your DACPAC publish profile by setting ExcludeObjectTypes=Types in the .publish.xml configuration file.