SetAlphamaps causing editor to crash V5.4.0f3

EDIT: It seems to work fine if I turn on Automatic Baking of Lightmaps in the lighting window. So problem solved :-)

Hey guys, I'm trying to add some procedural texture painting to the terrain in my project, and as a start I'm trying to get the example given in the Unity Documentation to work.

Here's my C# code:

 void Start()
     {
         // Get the attached terrain component
         Terrain terrain = GetComponent<Terrain>();
 
         // Get a reference to the terrain data
         TerrainData terrainData = terrain.terrainData;
 
         float[,,] map = new float[terrainData.alphamapWidth, terrainData.alphamapHeight, 5];
 
         // For each point on the alphamap...
         for (int y = 0; y < terrainData.alphamapHeight; y++)
         {
             for (int x = 0; x < terrainData.alphamapWidth; x++)
             {
                 // Get the normalized terrain coordinate that
                 // corresponds to the the point.
                 float normX = (x * 1 / (terrainData.alphamapWidth - 1));
                 float normY = (y * 1 / (terrainData.alphamapHeight - 1));
 
                 // Get the steepness value at the normalized coordinate.
                 float angle = terrainData.GetSteepness(normX, normY);
 
                 // Steepness is given as an angle, 0..90 degrees. Divide
                 // by 90 to get an alpha blending value in the range 0..1.
                 float frac = angle / 90;
 
                 map[x, y, 0] = frac;
                 map[x, y, 1] = 0;
                 map[x, y, 2] = 1-frac;
                 map[x, y, 3] = 0;
                 map[x, y, 4] = 0;
             }
         }
 
         terrainData.SetAlphamaps(0, 0, map);
     }

However this causes the editor to crash when I run the code. The only way I can get it to work is if I set the strength of the first texture to 1, like so:

 map[x, y, 0] = 1f;

Then it works but obviously doesn't result in the desired effect. Is there something really obvious I'm missing? I've spent hours googling for an answer but haven't found any solutions.

Thanks!