list all property names of controls based on condition in C#

I am trying to parse through all properties of all my controls (for example ascx files) from a certain page (this.Page.Controls) in order to obtain the name of the property that has a property value specified by me, for example - what is the name of the property that has the value "this is my header"? (it may very well be a textbox that contains this value).

The below returns this error:

Cannot get inner content of because the contents are not literal. System.Web.UI.HtmlControls.HtmlContainerControl.get_InnerHtml()

not sure what it means or how to correct it.

 public String searchMethod(List<Control> listOfControls, String searchedValue)
        {
            String result = "";

            foreach (var control in listOfControls)
            {
                PropertyInfo[] properties = control.GetType().GetProperties();
                foreach (PropertyInfo property in properties)
                {
                    if (property.PropertyType == typeof (string)) //added condition on this line *******************************
                    {
                        if (property.GetValue(control, null) != null)
                            if (property.GetValue(control, null).ToString().Contains("searched String"))
                            {
                                result = result + property.Name + "/" + property.GetValue(control, null) + "/";
                            }
                    }

                }
            }
            return result;
        }

I suppose it's stuck on a property that doesn't match the requirements, so why doesn't it simply move on to the next until it finds a good fit? Apparently the property that gets it stuck is a System.String Inner.Html

PS. I've tested the listOfControls fed to the method and it's being generated correctly

Later Update: Another method I am trying is:

public string Method1(List<Control> controlList, string propName)
        {
            string result = "";
            foreach (var control in controlList)
            {
                foreach(var prop in control.GetType().GetProperties())
                {
                    if(prop.PropertyType == typeof(string))
                    {
                        if((prop.GetValue(control,null).GetType()) == typeof(string))
                            if (prop.GetValue(control, null).ToString().Contains(propName))
                        result += prop.Name + "######";
                    }
                }
            }
            return result;
        }

but with this i get Object reference not set to an instance of an object. on line if((prop.GetValue(control,null).GetType()) == typeof(string))