# Controls Collection Cannot Be Modified because of Code Blocks Error Occurs
## Environment
Product
Progress® Telerik® UI for ASP.NET AJAX
## Description
When working with Telerik UI for ASP.NET AJAX, I get an error that the controls collection cannot be modified because the control contains code blocks.
## Error Message
`The Controls collection cannot be modified because the control contains code blocks`
## Cause
The possible cause for this issues is that the code block is inside the `head` tag, for example:
```ASP.NET
...
```
## Solution
To solve this issue, move the code block (`<% ... %>`) outside the `head` tag:
```ASP.NET
...
or
```