diff --git a/pyproject.toml b/pyproject.toml
index 9a56f3a..2448fc0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,7 +58,7 @@ dev = [
     "ruff",
     "pdbpp; sys_platform != 'win32'",
 ]
-docs = ["sphinx", "sphinx_rtd_theme", "sphinx-autodoc-typehints"]
+docs = ["numpy", "sphinx", "sphinx_rtd_theme", "sphinx-autodoc-typehints"]
 
 [project.urls]
 homepage = "https://github.com/funkelab/ilpy"