member initializer does not name a non-static data member or base class

member initializer does not name a non-static data member or base class

I'm having a hard time finding hits on google for this.
struct a {
float m_x;
float m_z;
public:
a(float x): m_x(x) {}
};
class b : public a {
b(float z): m_z(z) {}
};
On clang 3.2:
error: member initializer 'm_z' does not name a non-static data member or
base class
b(float z): m_z(z) {}