How to get FunctionType from CallInst when call is indirect in LLVM

If a function call is direct, you can get the Function type through the following code.

Function  * fun  = callInst->getCalledFunction();
Function  * funType = fun->getFunctionType();

However, if the call is indirect, that is, through a function pointer, the getCalledFunction returns NULL. So my question is how to get the Function type when a function is called through a function pointer.