Cannot deduce from type

[Chris via Digitalmars-d-learn]

Why is that?

import std.stdio, std.array

void main() {
   auto output = appender!(string);
   output ~= "world!";
//  output.data.insertInPlace(0, "Hello, ");  // Doesn't work
   auto asString = output.data;
   asString.insertInPlace(0, "Hello, ");  // Works
   writeln(asString);  // prints "Hello, world!"
}

Error: template std.array.insertInPlace cannot deduce function 
from argument types !()(string, int, string), candidates are:
.dvm/compilers/dmd-2.066.0/linux/bin/../../src/phobos/std/array.d(1031): 
        std.array.insertInPlace(T, U...)(ref T[] array, size_t 
pos, U stuff) if (!isSomeString!(T[]) && 
allSatisfy!(isInputRangeOrConvertible!T, U) && U.length > 0)
.dvm/compilers/dmd-2.066.0/linux/bin/../../src/phobos/std/array.d(1097): 
        std.array.insertInPlace(T, U...)(ref T[] array, size_t 
pos, U stuff) if (isSomeString!(T[]) && 
allSatisfy!(isCharOrStringOrDcharRange, U))