Troubleshooting Guide: SQL Server Error 4067 — CLR Type Serialization Failed (Invalid Cookie)

Why This Error Happens

This error typically occurs when a CLR user-defined type (UDT) or user-defined aggregate becomes corrupted or mismatched due to an incomplete assembly update, a version mismatch between the serialized binary data and the registered CLR type metadata, or improper ALTER ASSEMBLY operations that did not fully synchronize type definitions. It can also happen if the assembly was deployed with PERMISSION_SET = UNSAFE or EXTERNAL_ACCESS without properly registering the native serialization format, causing SQL Server to reject the data cookie during deserialization. In some cases, this is triggered by database migration or restoration from an older SQL Server version where the CLR type binary layout is incompatible with SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Verify CLR Integration is Properly Enabled

• Open SQL Server Management Studio (SSMS) and connect to your instance.
• Run the following to confirm CLR is enabled:

  EXEC sp_configure 'clr enabled';

• If disabled, enable it:

  EXEC sp_configure 'clr enabled', 1;
  RECONFIGURE;

Step 2: Rebuild and Redeploy the CLR Assembly

• Drop the affected assembly and any dependent types:

  DROP TYPE IF EXISTS dbo.YourCLRType;
  DROP ASSEMBLY IF EXISTS YourAssemblyName;

• Rebuild the .dll from source using Visual Studio 2022/2025 targeting the correct .NET Framework/CLR version supported by SQL Server 2025.
• Redeploy the assembly from a trusted local path, e.g.:

  CREATE ASSEMBLY YourAssemblyName
  FROM 'C:\SQLAssemblies\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;

• Recreate the CLR type:

  CREATE TYPE dbo.YourCLRType
  EXTERNAL NAME YourAssemblyName.[YourCLRType];

Step 3: Check for Version Mismatches After Migration or Restore

• If the database was migrated or restored from an older SQL Server version, confirm the assembly binary matches the current CLR hosting version in Windows Server 2025.
• Use the following query to inspect assembly metadata:

  SELECT * FROM sys.assemblies;
  SELECT * FROM sys.assembly_types;

• If mismatched, drop and redeploy the assembly using Step 2 instead of relying on the restored binary.

Step 4: Validate Windows and SQL Server Patch Level

• Confirm Windows Server 2025 / Windows 11 has the latest cumulative updates installed via Settings > Windows Update.
• Confirm SQL Server 2025 is patched to the latest Cumulative Update (CU) using:

  SELECT @@VERSION;

• Apply any pending CUs from the official Microsoft Download Center, as CLR serialization bugs are frequently addressed in these updates.

Alternative or Verification Steps

• After redeployment, run a test query that inserts and retrieves data using the CLR type (e.g., SELECT CAST(... AS dbo.YourCLRType)) to confirm serialization/deserialization completes without Error 4067.
• If the error persists, as a fallback, export the affected table data to a native SQL type (e.g., VARBINARY or NVARCHAR), drop the CLR type entirely, and rebuild the column using plain SQL types to bypass CLR serialization altogether.