diff --git a/requirements-optional.txt b/requirements-optional.txt
index 1128be57..df2145d1 100644
--- a/requirements-optional.txt
+++ b/requirements-optional.txt
@@ -8,6 +8,7 @@ phonopy
 #mayavi
 # Unit tests
 pytest
+pytest-cov
 pytest-xdist
 scripttest
 python-coveralls
\ No newline at end of file