syntex error

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[this line is inserted to help code formatting]

p=(2^3)
print "Prime field P=",p
F.<a> = GF(p);F
K.<b> = F.extension(x^2+(a^2+a+1)*x +a^2) ;K

when i run the code following error obtained on my macbook sage terminal and sage math cloud

Prime field P= 8
Finite Field in a of size 2^3

Error in lines 4-4
Traceback (most recent call last):
  File "/projects/sage/sage-7.3/local/lib/python2.7/site-packages/smc_sagews/sage_server.py", line 957, in execute
    exec compile(block+'\n', '', 'single') in namespace, locals
  File "", line 1, in <module>
  File "sage/rings/finite_rings/finite_field_base.pyx", line 1193, in sage.rings.finite_rings.finite_field_base.FiniteField.extension (/projects/sage/sage-7.3/src/build/cythonized/sage/rings/finite_rings/finite_field_base.c:12767)
    E = Field.extension(self, modulus, name=name, embedding=embedding)
  File "sage/rings/ring.pyx", line 1486, in sage.rings.ring.CommutativeRing.extension (/projects/sage/sage-7.3/src/build/cythonized/sage/rings/ring.c:13322)
    poly = poly.polynomial(self)
  File "sage/rings/polynomial/multi_polynomial.pyx", line 449, in sage.rings.polynomial.multi_polynomial.MPolynomial.polynomial (/projects/sage/sage-7.3/src/build/cythonized/sage/rings/polynomial/multi_polynomial.c:6868)
    raise ValueError("var must be one of the generators of the parent polynomial ring.")
***ValueError: var must be one of the generators of the parent polynomial ring.**