How to fix Error 5303 on Windows Server 2025 / Windows 11

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Why This Error Happens

This error occurs when SQL Server’s CLR integration engine attempts to apply a mutator method on a User-Defined Type (UDT) and the underlying .NET assembly returns a null reference instead of a valid object instance. This typically stems from a coding flaw in the custom CLR assembly where the Parse() or mutator method fails to properly instantiate the object before returning it. In rare cases, this can also be triggered by a corrupted or mismatched assembly binary that was deployed to SQL Server 2025 after a Windows Server update altered the .NET runtime binding.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Assembly and Type

  • Open SQL Server Management Studio (SSMS) and query sys.assemblies and sys.assembly_types to locate the specific CLR type referenced in the error message (%.*ls).
  • Run the following T-SQL to cross-reference the assembly:
SELECT a.name, t.name AS type_name 
FROM sys.assembly_types t
JOIN sys.assemblies a ON t.assembly_id = a.assembly_id;
  • Note the exact DLL path associated with this assembly for the next step.

Step 2: Audit the Source Code for the Mutator Method

  • Locate the Visual Studio project source for the UDT (usually a .cs file implementing IBinarySerialize).
  • Check the specific mutator method named in the error log; ensure it does not have a code path where the object reference is left uninitialized (e.g., return null; instead of return this;).
  • Recompile the assembly in Visual Studio 2022/2025 ensuring the target framework matches SQL Server 2025 CLR requirements (.NET Framework 4.8 or the updated .NET CLR strict security model).

Step 3: Redeploy the Assembly with UNSAFE Permissions (If Required)

  • If the UDT requires external resource access, verify the assembly permission set in SQL Server.
  • Drop and re-register the corrected assembly using the following commands in SSMS:
DROP ASSEMBLY YourAssemblyName;
GO
CREATE ASSEMBLY YourAssemblyName 
FROM 'C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\YourFile.dll' 
WITH PERMISSION_SET = SAFE;
GO
  • Ensure the file path points to the newly compiled DLL on the Windows Server 2025 host, not a cached or temporary build folder.

Step 4: Clear SQL Server Plan Cache

  • Since SQL Server 2025 caches execution plans for CLR type methods, a stale plan might still reference the old, broken binary logic.
  • Execute DBCC FREEPROCCACHE; to force SQL Server to recompile the execution path using the updated assembly.

Alternative or Verification Steps

  • Verification: Re-run the specific T-SQL transaction or stored procedure that triggered the original Event ID 5303. If the mutator executes without throwing an exception, the object instantiation logic has been successfully patched.
  • Alternative Fix: If recompiling the assembly is not immediately possible, wrap the calling T-SQL logic in a TRY...CATCH block to handle the SqlNullValueException gracefully, preventing the transaction from failing abruptly while you schedule a permanent code fix.