The header name format is invalid

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Hi,

I am getting inconsistent behaviour when adding a custom header to my HttpRequestMessage. When I add the header using a variable, I get System.FormatException: 'The header name format is invalid. When I explicitly define the header string (hard code it) it works. I can confirm the value passed to the variable is exactly the same. Here is my method, which may aid in reproducing:

        private static void AddSecurityRequestHeaders(HttpRequestMessage httpRequestMessage, string securityHeader, string securityValue)
        {
            httpRequestMessage.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue(Constants.Http.JsonMediaType));
            httpRequestMessage.Headers.Add(securityHeader, securityValue);
            //httpRequestMessage.Headers.Add("BusinessSecKey", "SecurityKeyValue");
        }

If you uncomment and use the section of code using the hard-coded header and value, it works.

If this is by design, can someone please explain to me what is going on?