Subelements of the symbol field must be strings of length 1 to 3 error

I am using wrann to write the symbols and the indices into the reference annotation file. I am getting this error Subelements of the symbol field must be strings of length 1 to 3 for the symbols list. I checked the type of the list elements using
for i in range(0,len(labels)):
print(isinstance(labels[i], str_types)) and the result is True for all the elements of the list. If I use a dummy symbol list I am able to write to the annotation file. Could you tell me what is it that I am doing wrong.