diff --git a/src/Data/ProtoLens/Compiler/Definitions.hs b/src/Data/ProtoLens/Compiler/Definitions.hs
index 9afb25a4..78d3f6a3 100644
--- a/src/Data/ProtoLens/Compiler/Definitions.hs
+++ b/src/Data/ProtoLens/Compiler/Definitions.hs
@@ -191,6 +191,9 @@ data FieldKind
         -- whether the field's FieldDescriptorProto of the field has
         -- options.map_entry set.
     | MapField MapEntryInfo
+       -- ^ A field containing a map of keys to values.
+       -- Serialized as a repeated field of an autogenerated "entry"
+       -- proto type, each instance of which contains a key-value pair.
 
 data FieldPacking
     = NotPackable  -- ^ Cannot be packed (e.g., strings or messages).