Added check that nu must be a scalar in MvStudentTRV

pymc/distributions/multivariate.py

@@ -262,6 +262,13 @@ class MvStudentTRV(RandomVariable):
     dtype = "floatX"
     _print_name = ("MvStudentT", "\\operatorname{MvStudentT}")
 
+    def make_node(self, rng, size, dtype, nu, mu, cov):
+        nu = at.as_tensor_variable(nu)
+        if not nu.ndim == 0:
+            raise ValueError("nu must be a scalar (ndim=0).")
+
+        return super().make_node(rng, size, dtype, nu, mu, cov)
+
     def __call__(self, nu, mu=None, cov=None, size=None, **kwargs):
 
         dtype = aesara.config.floatX if self.dtype == "floatX" else self.dtype

pymc/tests/test_distributions_random.py

@@ -13,6 +13,7 @@
 #   limitations under the License.
 import functools
 import itertools
+import re
 
 from typing import Callable, List, Optional
 
@@ -1115,8 +1116,20 @@ def mvstudentt_rng_fn(self, size, nu, mu, cov, rng):
         "check_pymc_params_match_rv_op",
         "check_pymc_draws_match_reference",
         "check_rv_size",
+        "test_errors",
     ]
 
+    def test_errors(self):
+        msg = "nu must be a scalar (ndim=0)."
+        with pm.Model():
+            with pytest.raises(ValueError, match=re.escape(msg)):
+                mvstudentt = pm.MvStudentT(
+                    "mvstudentt",
+                    nu=np.array([1, 2]),
+                    mu=np.ones(2),
+                    cov=np.full((2, 2), np.ones(2)),
+                )
+
 
 class TestMvStudentTChol(BaseTestDistribution):
     pymc_dist = pm.MvStudentT