How do I get rid of the following sign-conversion warning?

I am getting the following warning whenever the function initSetArray() is called :

error: conversion to ‘long unsigned int’ from ‘int’ may change the sign of the result [-Werror=sign-conversion]
  setarray = (set*)malloc(sizeof(set) * number_of_sets);   

The function initSetArray simply initializes the setarray.

void initSetArray(set *setarray, int number_of_sets, int number_of_blocks)
{
    setarray = (set*)malloc(sizeof(set) * number_of_sets);
}

I have defined two structures which are used in the helper function defined above:

typedef struct{
    uint64_t tag;   // For identifying the block
    uint8_t valid;  // Valid bit for the block
    uint8_t dirty;  // Dirty bit for the block
} block;

typedef struct{
    uint64_t index; // For identifying the set
    block* way;
} set;

I cannot exactly find out which variable is of type "long unsigned int". What can I do to resolve this issue?