add version dependency
diff --git a/instrumentation/README.ngram.md b/instrumentation/README.ngram.md
index de3ba43..da61ef3 100644
--- a/instrumentation/README.ngram.md
+++ b/instrumentation/README.ngram.md
@@ -10,8 +10,8 @@
Note that the original implementation (available
[here](https://github.com/bitsecurerlab/afl-sensitive))
is built on top of AFL's QEMU mode.
-This is essentially a port that uses LLVM vectorized instructions to achieve
-the same results when compiling source code.
+This is essentially a port that uses LLVM vectorized instructions (available from
+llvm versions 4.0.1 and higher) to achieve the same results when compiling source code.
In math the branch coverage is performed as follows:
`map[current_location ^ prev_location[0] >> 1 ^ prev_location[1] >> 1 ^ ... up to n-1`] += 1`
