diff --git a/README.md b/README.md
index d26da8f..6ed74ad 100644
--- a/README.md
+++ b/README.md
@@ -40,6 +40,8 @@ Create a new hash instance. `digestLength` defaults to `32`.
 
 Update the hash with a new piece of data. `data` should be a buffer or uint8array.
 
+The size of the data is limited to `65536000 - 64 - 216*n` bytes, where `n` is the number of Blake2b instances.
+
 #### `var digest = hash.digest([enc])`
 
 Digest the hash.
diff --git a/index.js b/index.js
index 26ea867..6f50b64 100644
--- a/index.js
+++ b/index.js
@@ -79,6 +79,7 @@ Blake2b.prototype._realloc = function (size) {
 Blake2b.prototype.update = function (input) {
   assert(this.finalized === false, 'Hash instance finalized')
   assert(input instanceof Uint8Array, 'input must be Uint8Array or Buffer')
+  assert(input.length + head <= 65536000, 'input + head must be of size 65,536,000 or less')
 
   if (head + input.length > this._memory.length) this._realloc(head + input.length)
   this._memory.set(input, head)