heegner_index() output

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Consider the elliptic curve E=EllipticCurve('37a')

This elliptic curve has rank 1 over $\mathbb{Q}$. I am interested in knowing which primes divide the index of the Heegner point $y_K$ in the group $E(K)$ modulo torsion where $K=\mathbb{Q}(\sqrt{-7})$

Since $E$ has rank 1, heegner_index_bound() does not work so I try E.heegner_index(-7). The output is 1.00000?

This output should be an interval. I do not understand the output. What does the question mark mean?