diff --git a/pypfopt/hierarchical_portfolio.py b/pypfopt/hierarchical_portfolio.py
index 6dc76d07..248aac15 100644
--- a/pypfopt/hierarchical_portfolio.py
+++ b/pypfopt/hierarchical_portfolio.py
@@ -176,9 +176,6 @@ def optimize(self, linkage_method="single"):
         OrderedDict
             weights for the HRP portfolio
         """
-        if linkage_method not in sch._LINKAGE_METHODS:
-            raise ValueError("linkage_method must be one recognised by scipy")
-
         if self.returns is None:
             cov = self.cov_matrix
             corr = cov_to_corr(self.cov_matrix).round(6)
diff --git a/requirements.txt b/requirements.txt
index 7c90d815..1b8a9410 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,6 @@
 cvxpy>=1.1.19
-numpy>=1.0.0
-pandas>=0.19
+numpy>=1.26.0,<3.0.0
+pandas>=1.0.0,<4.0.0
 scikit-base<0.14.0
 scikit-learn>=0.24.1
 scipy>=1.3.0