Annex F (informative) Core undefined behavior [ub]

F.3 [expr]: Expressions [ub.expr]

F.3.27[ub:expr.mptr.oper.member.func.null]
Specified in: [expr.mptr.oper]

If the second operand in a .* expression is the null member pointer value ([conv.mem]), the behavior is undefined.
[Example 1: struct S { int f(); }; void f() { S cs; int (S::*pm)() = nullptr; (cs.*pm)(); // undefined behavior, the second operand is null } — end example]