Struggling with struct.unpack() and "p" format specifier

[Geoffrey]

Hope someone can help.
I am trying to read data from a file binary file and then unpack the
data into python variables.  Some of the data is store like this;

xbuffer: '\x00\x00\xb9\x02\x13EXCLUDE_CREDIT_CARD'
# the above was printed using repr(xbuffer).  
# Note that int(0x13) = 19 which is exactly the length of the visible
text
#

In the code I have the following statement;
x = st.unpack('>xxBBp',xbuffer)

This throws out the following error;

x = st.unpack('>xxBBp',xbuffer)
error: unpack str size does not match format

As I read the documentation the "p" format string seems to address
this situation, where the number bytes of the string to read is the
first byte of the stored value but I keep getting this error.

Am I missing something ?
Can the "p" format character be used to unpack this type of data ?

As I mentioned, I can parse the string and read it with multiple
statements, I am just looking for a more efficient solution.

Thanks.