It looks like the Microsoft developers don't allow the IPA pronunciation 'ɡ' character but allows the English language 'g' character. The IPA language 'ɡ' character is a two byte character with value: 609, whereas the English character 'g' has value: 67. This bug causes a valid SSML to throw NULL value error in the .NET Speech API. The valid IPA can be found at Wikipedia, see the Bangalore IPA (ˈbæŋɡəloːɾ)