Use basis._symbol as prefix only when it is a string

src/sage/tensor/modules/finite_rank_free_module.py

@@ -2555,6 +2555,15 @@ def isomorphism_with_fixed_basis(self, basis, codomain=None):
             sage: phi_e(e[1] + 2 * e[2])
             e[1] + 2*e[2]
 
+            sage: abc = V.basis(['a', 'b', 'c'], symbol_dual=['d', 'e', 'f']); abc
+            Basis (a,b,c) on the 3-dimensional vector space over the Rational Field
+            sage: phi_abc = V.isomorphism_with_fixed_basis(abc); phi_abc
+            Generic morphism:
+            From: 3-dimensional vector space over the Rational Field
+            To:   Free module generated by {1, 2, 3} over Rational Field
+            sage: phi_abc(abc[1] + 2 * abc[2])
+            B[1] + 2*B[2]
+
         Providing a codomain::
 
             sage: W = CombinatorialFreeModule(QQ, ['a', 'b', 'c'])
@@ -2578,14 +2587,16 @@ def isomorphism_with_fixed_basis(self, basis, codomain=None):
             Traceback (most recent call last):
             ...
             ValueError: domain and codomain must have the same base ring
-
         """
         base_ring = self.base_ring()
         if codomain is None:
             from sage.combinat.free_module import CombinatorialFreeModule
-
+            if isinstance(basis._symbol, str):
+                prefix = basis._symbol
+            else:
+                prefix = None
             codomain = CombinatorialFreeModule(base_ring, list(self.irange()),
-                                               prefix=basis._symbol)
+                                               prefix=prefix)
         else:
             if codomain.rank() != self.rank():
                 raise ValueError("domain and codomain must have the same rank")