How do i define the label tag for this field? I have given the sample code below,

Code HTML4Strict:
<label for="PrefStartMonth" class="lbltxt1">Preferred start month/ year:</label>
<input type="text" id="PrefStartMonth" name="PrefStartMonth" size="10" maxlength="12"
class="txtbox5" /> /
<input type="text" id="PrefStartYear" name="PrefStartYear" size="4" maxlength="4" class="txtbox3" />
<label class="lbltxt1">Citizenship:</label> <input type="radio" id="USCitizen" name="Citizenship" value="US Citizen" /> <label for="USCitizen">US Citizen</label>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <input type="radio" id="NaturalizedUSCitizen" name="Citizenship" value="Naturalized US Citizen" /> <label for="NaturalizedUSCitizen" >Naturalized US Citizen</label> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <input type="radio" id="PermanentResident" name="Citizenship" value="Permanent Resident" /> <label for="PermanentResident">Permanent Resident</label>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <input type="radio" id="NonUS" name="Citizenship" value="Not a US Citizen" /> <label for="NonUS" >Not a US Citizen</label>

Now i have to go and checked the section 503 i got the error message. The message was shown below,

[WCAG v1 12.4 (AA), US-508-n] No matching <label> tag could be found, or label is empty

I have define the id('PrefStartMonth') for the first input tag. These input tags defined one label element so i got the error message. How do i solve it these kind of fields.