diff --git a/pyproject.toml b/pyproject.toml
index 2f03d17..e8680c2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -20,6 +20,7 @@ maintainers = [
 ]
 dependencies = [
     'numpy',
+    'numpy-quaternion',
     'astropy>=2.0',
     'scipy>=1.0',
     'matplotlib>=2.0',