How do I change the \mathtt font with mathspec?

There seems to be a bug in mathspec; if I ask for

\showthe\mathcode`1

I get 1841, i.e., "0731. It seems that 7 is the math group for Gotham Narrow Light, so the mathcode should be "7731. Indeed, if I say

\mathcode`\0=\numexpr\mathcode`0+"7000\relax
\mathcode`\1=\numexpr\mathcode`1+"7000\relax
\mathcode`\2=\numexpr\mathcode`2+"7000\relax
\mathcode`\3=\numexpr\mathcode`3+"7000\relax
\mathcode`\4=\numexpr\mathcode`4+"7000\relax
\mathcode`\5=\numexpr\mathcode`5+"7000\relax
\mathcode`\6=\numexpr\mathcode`6+"7000\relax
\mathcode`\7=\numexpr\mathcode`7+"7000\relax
\mathcode`\8=\numexpr\mathcode`8+"7000\relax
\mathcode`\9=\numexpr\mathcode`9+"7000\relax

I get the desired behavior.