diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index ccc04bc..0ef964d 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -228,7 +228,7 @@
  */
 enum GrMaskFormat {
     kA8_GrMaskFormat,    //!< 1-byte per pixel
-    kA565_GrMaskFormat,  //!< 2-bytes per pixel
+    kA565_GrMaskFormat,  //!< 2-bytes per pixel, RGB represent 3-channel LCD coverage
     kARGB_GrMaskFormat,  //!< 4-bytes per pixel, color format
 
     kLast_GrMaskFormat = kARGB_GrMaskFormat
@@ -239,12 +239,15 @@
  *  Return the number of bytes-per-pixel for the specified mask format.
  */
 static inline int GrMaskFormatBytesPerPixel(GrMaskFormat format) {
-    SkASSERT((unsigned)format <= 3);
+    SkASSERT((unsigned)format < kMaskFormatCount);
     // kA8   (0) -> 1
     // kA565 (1) -> 2
-    // kARGB (3) -> 4
+    // kARGB (2) -> 4
     static const int sBytesPerPixel[] = { 1, 2, 4 };
     SK_COMPILE_ASSERT(SK_ARRAY_COUNT(sBytesPerPixel) == kMaskFormatCount, array_size_mismatch);
+    SK_COMPILE_ASSERT(kA8_GrMaskFormat == 0, enum_order_dependency);
+    SK_COMPILE_ASSERT(kA565_GrMaskFormat == 1, enum_order_dependency);
+    SK_COMPILE_ASSERT(kARGB_GrMaskFormat == 2, enum_order_dependency);
 
     return sBytesPerPixel[(int) format];
 }