diff --git a/gtda/homology/tests/test_simplicial.py b/gtda/homology/tests/test_simplicial.py
index fbc2baa8c..c9d2926aa 100644
--- a/gtda/homology/tests/test_simplicial.py
+++ b/gtda/homology/tests/test_simplicial.py
@@ -131,6 +131,15 @@ def test_wrp_params():
         wrp.fit_transform(X_pc)
 
 
+def test_wrp_metric_params():
+    def metric(x, y, **kwargs):
+        return np.linalg.norm(x - y)
+
+    metric_params = {"parameter": 0.}
+    wrp = WeightedRipsPersistence(metric=metric, metric_params=metric_params)
+    wrp.fit_transform(X_pc)
+
+
 def test_wrp_not_fitted():
     wrp = WeightedRipsPersistence()
 
diff --git a/gtda/utils/validation.py b/gtda/utils/validation.py
index 32cfa6872..37929b140 100644
--- a/gtda/utils/validation.py
+++ b/gtda/utils/validation.py
@@ -138,7 +138,10 @@ def _validate_params_single(_parameter, _reference, _name):
         ref_type = _validate_params_single(parameter, reference, name)
         if ref_type:
             ref_of = reference.get('of', None)
-            if ref_type == dict:
+            if ref_of is None:
+                # if ref_of is None, the elements are not to be validated
+                continue
+            elif ref_type == dict:
                 _validate_params(parameter, ref_of, rec_name=name)
             else:  # List, tuple or ndarray type
                 for i, parameter_elem in enumerate(parameter):