What does this particular Visual Studio web error message mean when it says, "The ControlToValidate property of 'rfvPhone' cannot be blank?"

I'm working on a basic visual studio project using web development. When I run my code, I get this weird web error message. There was no error message indicating any errors in Visual Studio. Could it be something else I missed?

Server Error in '/' Application.

The ControlToValidate property of 'rfvPhone' cannot be blank. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: The ControlToValidate property of 'rfvPhone' cannot be blank.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. 

Stack Trace: 


[HttpException (0x80004005): The ControlToValidate property of 'rfvPhone' cannot be blank.]
   System.Web.UI.WebControls.BaseValidator.ControlPropertiesValid() +9805468
   System.Web.UI.WebControls.BaseValidator.OnPreRender(EventArgs e) +38
   System.Web.UI.Control.PreRenderRecursiveInternal() +90
   System.Web.UI.Control.PreRenderRecursiveInternal() +163
   System.Web.UI.Control.PreRenderRecursiveInternal() +163
   System.Web.UI.Control.PreRenderRecursiveInternal() +163
   System.Web.UI.Control.PreRenderRecursiveInternal() +163
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +906


Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.8.4042.0