How to fix Error 4152 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4152 — “Type is not a CLR type”

Why This Error Happens

This error occurs when SQL Server attempts to reference, assign, or cast a database type that was expected to be a CLR (Common Language Runtime) user-defined type, but the type is actually a native SQL type or does not exist in the CLR type catalog. This commonly happens after a database restore or migration where the CLR assembly backing the type was not properly registered, or when a type name collision occurs between a native alias type and a CLR type sharing the same name. It can also result from a corrupted or missing assembly binary that SQL Server 2025 cannot resolve to the metadata entry in sys.assembly_types.

Step-by-Step Resolution Guide

Step 1: Verify the Type Registration in the Database

  • Run the following query to confirm whether the type is registered as a CLR type:
  SELECT name, is_assembly_type FROM sys.types WHERE name = 'YourTypeName';
  • If is_assembly_type returns 0, the type exists but is not linked to a CLR assembly — this confirms the root cause.
  • Cross-check against sys.assembly_types to see if the assembly binding is missing.

Step 2: Re-register or Re-deploy the CLR Assembly

  • Confirm CLR integration is enabled on the instance:
  EXEC sp_configure 'clr enabled', 1;
  RECONFIGURE;
  • Drop and recreate the affected type binding to the assembly:
  DROP TYPE dbo.YourTypeName;
  CREATE TYPE dbo.YourTypeName EXTERNAL NAME YourAssembly.[YourClassName];
  • Ensure the .dll assembly file used during deployment matches the version originally compiled against the database schema.

Step 3: Check for Assembly Corruption or Version Mismatch

  • Navigate to the SQL Server binaries directory (commonly C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\) and confirm the assembly file hasn’t been altered or deleted post-upgrade to SQL Server 2025.
  • Re-import the assembly using:
  ALTER ASSEMBLY YourAssembly FROM 'C:\Path\To\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Restart the SQL Server service via services.msc or:
  Restart-Service -Name "MSSQLSERVER"

Step 4: Resolve Naming Conflicts Between Native and CLR Types

  • Query sys.types for duplicate type names across schemas:
  SELECT name, schema_id, is_assembly_type FROM sys.types WHERE name = 'YourTypeName';
  • Rename or fully schema-qualify the CLR type reference in your T-SQL code (e.g., dbo.MyClrType instead of just MyClrType) to eliminate ambiguity with system or native alias types.

Alternative or Verification Steps

  • After re-deployment, test by declaring a variable of the type (DECLARE @x dbo.YourTypeName;) — if no error is thrown, the CLR type binding is successfully restored.
  • If the issue persists, drop the assembly entirely (DROP ASSEMBLY YourAssembly) and perform a clean redeployment from source using CREATE ASSEMBLY ... FROM 'path' WITH PERMISSION_SET = SAFE, then recreate all dependent types and functions.