How to fix Error 4174 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4174 — Delayed CLR Type Instantiation Fetch Value Query

Why This Error Happens

This error occurs when a query attempts to retrieve a value from a CLR user-defined type (UDT) or large object (LOB) column using delayed/lazy fetch semantics, but the underlying column reference does not resolve to a valid large value type at execution time. This typically happens due to a mismatch between the assembly’s serialization format (native vs. user-defined) and the column metadata stored in the database, often triggered after a CLR assembly update, an in-place upgrade to SQL Server 2025, or corrupted type metadata in the system catalog. It can also surface when a query plan caches a stale reference to a CLR type that has since been altered or dropped and recreated with different properties.

Step-by-Step Resolution Guide

Step 1: Verify CLR Integration and Assembly Status

  • Run SELECT * FROM sys.assemblies; to confirm the CLR assembly backing the UDT is marked as is_visible = 1 and not in a faulted state.
  • Check sp_configure 'clr enabled'; to ensure CLR integration is active (value should be 1).
  • Run ALTER ASSEMBLY <AssemblyName> WITH REBUILD; if the assembly shows signs of corruption or version mismatch.

Step 2: Clear and Rebuild the Query Plan Cache

  • Execute DBCC FREEPROCCACHE; to remove potentially stale execution plans referencing outdated CLR type metadata.
  • Run DBCC FREESYSTEMCACHE('ALL'); to flush system-level metadata caches tied to UDT definitions.
  • Re-run the failing query immediately after to see if the error persists with a freshly compiled plan.

Step 3: Recreate the CLR Type Column Reference

  • Identify the affected column using SELECT name, system_type_id, user_type_id FROM sys.columns WHERE user_type_id > 256; to isolate CLR-based columns.
  • Script out the table, drop the column referencing the CLR UDT, and recreate it using ALTER TABLE <TableName> ADD <ColumnName> <UDTName>; to force fresh metadata binding.
  • Reload or re-cast the data into the new column using UPDATE statements with explicit CONVERT or CAST operations.

Step 4: Update SQL Server 2025 Cumulative Package

  • Navigate to Windows Update or the SQL Server Installation Center on Windows Server 2025 / Windows 11 and check for the latest Cumulative Update (CU) under Microsoft SQL Server 2025 Setup.
  • Apply the update via Setup.exe /Action=Patch /IAcceptSQLServerLicenseTerms /Quiet from an elevated PowerShell session if deploying silently across multiple nodes.
  • Restart the SQL Server service (net stop MSSQLSERVER && net start MSSQLSERVER) after patching completes.

Alternative or Verification Steps

  • Re-run the original query with SET STATISTICS XML ON; to confirm the execution plan no longer references a broken CLR serialization path, and validate the result set returns the correct large value data.
  • If the error persists, isolate the issue by querying the UDT column in isolation (SELECT <Column>.ToString() FROM <Table>) rather than as part of a larger JOIN or computed expression, as this confirms whether the root cause is plan-level or assembly-level.