How to fix Error 6948 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 6948 – “Invalid Type Definition for Type ‘%s'” in SQL Server 2025

Why This Error Happens

Error 6948 occurs when SQL Server encounters a corrupted, mismatched, or incompatible CLR (Common Language Runtime) user-defined type (UDT) or user-defined aggregate during a data operation. This is typically caused by a mismatch between the assembly version registered in the database and the actual implementation loaded into memory, often following an in-place upgrade to SQL Server 2025 or a Windows Server 2025 patch that updated the .NET runtime. It can also occur if system catalog metadata referencing the type has become corrupted due to improper assembly deployment or an interrupted ALTER ASSEMBLY operation.

Step-by-Step Resolution Guide

Step 1: Identify the Affected Type and Assembly

  • Run the following query to locate the problematic type: SELECT name, assembly_id FROM sys.types WHERE is_assembly_type = 1;
  • Cross-reference the assembly_id against sys.assemblies and sys.assembly_files to confirm the binary is still present and valid.
  • Check the SQL Server Error Log (Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log) for the exact timestamp and object ID associated with the 6948 error.

Step 2: Validate and Re-register the CLR Assembly

  • Confirm “clr strict security” and “clr enabled” configurations are correctly set via: EXEC sp_configure 'clr enabled', 1; RECONFIGURE;
  • Drop and re-add the assembly reference to force SQL Server to reload the binary definition:
  • ALTER ASSEMBLY [YourAssemblyName] FROM 'C:\Path\To\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Ensure the DLL file permissions allow the SQL Server service account (NT Service\MSSQLSERVER) read access on Windows Server 2025.

Step 3: Repair System Catalog Corruption

  • If re-registering the assembly does not resolve the metadata mismatch, run DBCC CHECKDB('YourDatabaseName') WITH DATA_PURITY; to check for underlying catalog corruption.
  • If corruption is confirmed in system tables related to the type definition, restore the database from the last known good backup taken prior to the error’s first occurrence.
  • Avoid using REPAIR_ALLOW_DATA_LOSS on user-defined type corruption unless no valid backup exists, as it may force-drop the type definition entirely.

Step 4: Rebuild the Type in a Test Environment

  • If the assembly source code is available, recompile the DLL using the .NET SDK version matched to SQL Server 2025 CLR integration requirements.
  • Deploy the newly compiled assembly to a staging instance first to confirm the CREATE TYPE... EXTERNAL NAME command executes without throwing Error 6948.
  • Script out dependent objects (tables using the UDT as a column) before dropping and recreating the type in production.

Alternative or Verification Steps

  • Verify the fix by querying a table that uses the specific UDT column (e.g., SELECT TOP 10 * FROM dbo.YourTable;) and confirming data serializes/deserializes without throwing the error.
  • If the error persists across all repair attempts, the safest alternative is to export the data using the type’s string representation (via ToString() methods) into a staging table, drop the UDT-dependent objects entirely, and migrate the data to a standard SQL data type (e.g., VARBINARY or NVARCHAR) to bypass the CLR dependency.