diff --git a/pymc/model/fgraph.py b/pymc/model/fgraph.py
index 404df717c2..805208d7e0 100644
--- a/pymc/model/fgraph.py
+++ b/pymc/model/fgraph.py
@@ -375,10 +375,10 @@ def first_non_model_var(var):
             [var] = model_var.owner.inputs
             model.data_vars.append(var)
         else:
-            raise TypeError(f"Unexpected ModelVar type {type(model_var)}")
+            raise TypeError(f"Unexpected ModelVar type {type(op)}")
 
         var.name = op.name
-        dims = list(op.dims) if op.dims else None
+        dims = tuple(op.dims) if op.dims else None
         model.add_named_variable(var, dims=dims)
 
     return model
diff --git a/tests/model/test_fgraph.py b/tests/model/test_fgraph.py
index 13a1a3e9de..86d9274e2d 100644
--- a/tests/model/test_fgraph.py
+++ b/tests/model/test_fgraph.py
@@ -61,7 +61,8 @@ def test_basic():
 
     assert m_new.coords == {"test_dim": tuple(range(3))}
     assert m_new._dim_lengths["test_dim"].eval() == 3
-    assert m_new.named_vars_to_dims == {"z": ["test_dim"]}
+    assert m_new.named_vars_to_dims == {"z": ("test_dim",)}
+    assert m_new.named_vars_to_dims == m_old.named_vars_to_dims
 
     named_vars = {"x", "y", "w", "z", "pot"}
     assert set(m_new.named_vars) == named_vars
@@ -353,9 +354,9 @@ def test_fgraph_rewrite(non_centered_rewrite):
 
     m_new = model_from_fgraph(fg)
     assert m_new.named_vars_to_dims == {
-        "subject_mean": ["subject"],
-        "subject_mean_raw_": ["subject"],
-        "obs": ["subject"],
+        "subject_mean": ("subject",),
+        "subject_mean_raw_": ("subject",),
+        "obs": ("subject",),
     }
     assert set(m_new.named_vars) == {
         "group_mean",
diff --git a/tests/model/transform/test_conditioning.py b/tests/model/transform/test_conditioning.py
index 5f8eb99ee4..d4c2235135 100644
--- a/tests/model/transform/test_conditioning.py
+++ b/tests/model/transform/test_conditioning.py
@@ -132,7 +132,8 @@ def test_observe_dims():
         x = pm.Normal("x", dims="test_dim")
 
     m_new = observe(m_old, {x: np.arange(5, dtype=config.floatX)})
-    assert m_new.named_vars_to_dims["x"] == ["test_dim"]
+    assert m_new.named_vars_to_dims["x"] == ("test_dim",)
+    assert m_new.named_vars_to_dims == m_old.named_vars_to_dims
 
 
 def test_do():
@@ -229,13 +230,15 @@ def test_do_dims():
         m,
         {"x": np.zeros(10, dtype=config.floatX)},
     )
-    assert do_m.named_vars_to_dims["x"] == ["test_dim"]
+    assert do_m.named_vars_to_dims["x"] == ("test_dim",)
+    assert do_m.named_vars_to_dims == m.named_vars_to_dims
 
     do_m = do(
         m,
         {"y": np.zeros(10, dtype=config.floatX)},
     )
-    assert do_m.named_vars_to_dims["y"] == ["test_dim"]
+    assert do_m.named_vars_to_dims["y"] == ("test_dim",)
+    assert do_m.named_vars_to_dims == m.named_vars_to_dims
 
 
 @pytest.mark.parametrize("prune", (False, True))