How to fix Error 6953 on Windows Server 2025 / Windows 11

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Microsoft SQL Server 2025 Troubleshooting Guide: Error 6953

Why This Error Happens

Error 6953 (“Invalid type definition for type ‘%s'”) typically occurs when SQL Server encounters a corrupted or malformed CLR (Common Language Runtime) user-defined type or table-valued type definition in the system catalog. This can result from an incomplete assembly deployment, a mismatch between the assembly binary and its metadata after an in-place upgrade to SQL Server 2025, or corruption in the underlying system tables (sys.assembly_types). It may also surface when a database is migrated from an older SQL Server version without properly re-registering CLR assemblies on Windows Server 2025 / Windows 11.

Step-by-Step Resolution Guide

Step 1: Verify the Affected Type and Assembly Status

  • Run the following query to identify the problematic type: SELECT name, assembly_id, is_table_type FROM sys.assembly_types WHERE name = '<type_name>';
  • Cross-check the associated assembly using SELECT * FROM sys.assemblies WHERE assembly_id = <id>;
  • Confirm whether the assembly binary exists and is accessible at its original deployment path (check sys.assembly_files).

Step 2: Rebuild or Re-register the CLR Assembly

  • Open SQL Server Management Studio (SSMS) 2025 as Administrator.
  • Drop the corrupted type and dependent objects: DROP TYPE dbo.<type_name>;
  • Drop and re-add the assembly: DROP ASSEMBLY <assembly_name>; ALTER DATABASE <db_name> SET TRUSTWORTHY ON; CREATE ASSEMBLY <assembly_name> FROM 'C:\SQLAssemblies\<file>.dll' WITH PERMISSION_SET = SAFE;
  • Recreate the user-defined type referencing the restored assembly.

Step 3: Validate System Catalog Integrity

  • Run DBCC CHECKDB ('<database_name>') WITH DATA_PURITY; to rule out catalog or page-level corruption.
  • If corruption is confirmed, restore the affected database from the most recent verified backup taken before the upgrade.
  • Ensure the SQL Server 2025 service account has Read/Execute permissions on the assembly’s source folder under C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\.

Step 4: Update SQL Server 2025 Cumulative Update

  • Check your current build via SELECT @@VERSION;
  • Download and apply the latest Cumulative Update (CU) from the Microsoft Update Catalog, since known catalog metadata bugs are often patched in CUs.
  • Reboot Windows Server 2025 / Windows 11 and restart the SQL Server service after installation.

Alternative or Verification Steps

  • After applying fixes, re-run the original query or stored procedure that triggered Error 6953 to confirm the type resolves correctly without exceptions.
  • If the error persists, export the schema and data to a new database using the SQL Server Import/Export Wizard, bypassing the corrupted type definition entirely.