diff --git a/test/test_specs.py b/test/test_specs.py
index 25aec510c94..5cc2ed97226 100644
--- a/test/test_specs.py
+++ b/test/test_specs.py
@@ -1907,6 +1907,22 @@ def test_unboundeddiscrete(
         assert spec == torch.stack(spec.unbind(0), 0)
         assert spec == torch.stack(spec.unbind(-1), -1)
 
+    def test_composite_encode_err(self):
+        c = CompositeSpec(
+            a=UnboundedContinuousTensorSpec(
+                1,
+            ),
+            b=UnboundedContinuousTensorSpec(
+                2,
+            ),
+        )
+        with pytest.raises(KeyError, match="The CompositeSpec instance with keys"):
+            c.encode({"c": 0})
+        with pytest.raises(
+            RuntimeError, match="raised a RuntimeError. Scroll up to know more"
+        ):
+            c.encode({"a": 0, "b": 0})
+
 
 @pytest.mark.parametrize(
     "device",
diff --git a/torchrl/data/tensor_specs.py b/torchrl/data/tensor_specs.py
index a6ccf0dbeaf..42fc851d2a6 100644
--- a/torchrl/data/tensor_specs.py
+++ b/torchrl/data/tensor_specs.py
@@ -3979,6 +3979,10 @@ def encode(
                 raise KeyError(
                     f"The CompositeSpec instance with keys {self.keys()} does not have a '{key}' key."
                 )
+            except RuntimeError as err:
+                raise RuntimeError(
+                    f"Encoding key {key} raised a RuntimeError. Scroll up to know more."
+                ) from err
         return out
 
     def __repr__(self) -> str: