STL nested containers dereferencing error

I'm pretty rusty in my C++ and what little STL knowledge I once had. I'm particularly struggling to read the voluminous error messages generated.

Given:

typedef map<string,int>layerType;
 typedef vector<layerType> aggregateLayersType;
 

What's wrong with:

bool LayerManager::use_layers(aggregateLayersType& layers)
 {
   int layerVal = layers[0]["ts"];
 } 
 

The error is:

> No viable overloaded operator[] for type
 > 'std::__debug::map<std::basic_string<char, std::char_traits<char>,
 > std::allocator<char> >, int, std::less<std::basic_string<char,
 > std::char_traits<char>, std::allocator<char> > >,
 > std::allocator<std::pair<const std::basic_string<char,
 > std::char_traits<char>, std::allocator<char> >, int> > >'
 

I'm sure it's going to be something simple once someone points out the obvious.

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