Arduino/ESP8266 no SPI data coming from MCP3008

I'm using an ESP8266 (NodeMCU Dev Kit v1) to read analog data via an MCP3008 ADC chip.

However I'm receiving only zeros when I know non-zero data should be present. I'm using the following code, which works perfectly when using the MCP3008 with my Arduino UNO. Using verbose mode on compilation, I can confirm that when compiling for the ESP8266, that is indeed linking the ESP8266 SPI library.

Am I missing something here in the code?

#include <SPI.h>

#define CS_PIN 15 // Use 10 when compiling for Arduino

int adcRead(int channel) {
  if ((channel > 7) || (channel < 0)) {
    return -1;
  }

  digitalWrite(CS_PIN, LOW);

  SPI.transfer(1);
  uint8_t r1 = SPI.transfer((channel + 8) << 4);
  uint8_t r2 = SPI.transfer(0);

  digitalWrite(CS_PIN, HIGH);

  return ((r1 & 3) << 8) + r2;
}

void setup() {
  SPI.begin();
  SPI.setBitOrder(MSBFIRST);
  SPI.setDataMode(SPI_MODE0);
  SPI.setFrequency(1000000); // 1 MHz -- remove line when running on Arduino UNO
  Serial.begin(9600);
  digitalWrite(CS_PIN, HIGH);
}

void loop() {
  Serial.println(adcRead(0));
  delay(50);
}

In terms of wiring, I have the following

MCP3008   ESP8266
V_DD      3v3
V_REF     3v3
AGND      GND
CLK       GPIO14 (D5)
D_OUT     GPIO12 (D6)
D_IN      GPIO13 (D7)
CS        GPIO15 (D8)
DGND      GND