Unable to allocate memory with k_malloc

Question

nRF Connect SDK 2.3.0 on nRF52840DK 
 I am trying to add a fifo to my application but when trying to allocate the memory for the data item, k_malloc always returns NULL no matter what I do. The data item I can confirm a size of 8, and my CONFIG_HEAP_MEM_POOL_SIZE is set to 4096 in my config file. 
 K_FIFO_DEFINE(my_fifo);

struct data_item_t {
	void *fifo_reserved;
	uint32_t data;
};

static void uart_cb(const struct device *dev, struct uart_event *evt, void *user_data)
{
	uint32_t* rsp_p = (uint32_t *) evt->data.rx.buf;
	
	int bytes_written;
	switch (evt->type) {

	case UART_RX_RDY:

 //Get controller input response from UART RX buffer
 struct data_item_t *buf = k_malloc(sizeof(*buf));//TODO k_malloc returns NULL
 
 uint32_t rsp_32 = *rsp_p;
 if (buf == NULL){
 LOG_INF("Unable to allocate memory");
 break;
 }
 int bytes_written = snprintf(&buf->data, 4, "%x", rsp_32);
 k_fifo_put(&my_fifo, buf);


 break;
	case UART_RX_DISABLED:
 uart_rx_enable(dev ,rx_buf,sizeof rx_buf,RECEIVE_TIMEOUT);
 break;
 
	default:
 break;
	}
}