How to Solve Functional Differential Equation

I tried to solve this equation numerically.

eqnEx = x''[t] + x[2 t] == 0; NDSolve[{eqnEx, x[1] == 10, x'[1] == 0}, x[t], {t, 1, 10}] 

Two important character of the equation are that function variable is 2t and initial condition is given at t=1 .

After Calculation, Mathematica gives this error message

NDSolve : The method currently implemented for delay differential equations does not support delays that depend directly on the time variable or dependent variables 

After google, I find these kind of equation are called Functional Differential Equation and I could not obtain how to solve it. Is there a way to solve these kind of equation numerically?