diff --git a/edg/core/Blocks.py b/edg/core/Blocks.py
index 7de9c37c2..8a23dc024 100644
--- a/edg/core/Blocks.py
+++ b/edg/core/Blocks.py
@@ -219,7 +219,7 @@ def __init__(self, ref: ConstraintExpr, units: str):
       self.ref = ref
       self.units = units
 
-    def set_elt_proto(self, pb, ref_map):
+    def set_elt_proto(self, pb, ref_map=None):
       new_phrase = pb.description.add()
       new_phrase.param.path.CopyFrom(ref_map[self.ref])
       new_phrase.param.unit = self.units
@@ -554,7 +554,10 @@ def connect(self, *connects: Union[BasePort, Connection], flatten=False) -> Conn
         enclosing_block = port._block_parent()
         assert enclosing_block is not None
         if enclosing_block._parent is not self:
-          raise UnconnectableError("Inaccessible port for connection")
+          raise UnconnectableError("Inaccessible port: must be own port or inner block port")
+        if not(port._parent is enclosing_block or \
+                (isinstance(port._parent, Vector) and port._parent._parent is enclosing_block)):
+          raise UnconnectableError("Inaccessible port: cannot connect inner ports on inner blocks")
       self._connects_by_port[port] = connect
     connect.add_ports(connects_ports_new)
 
diff --git a/edg/core/test_bundle.py b/edg/core/test_bundle.py
index 68e5369a4..e22bf4f0b 100644
--- a/edg/core/test_bundle.py
+++ b/edg/core/test_bundle.py
@@ -1,4 +1,3 @@
-from typing import *
 import unittest
 
 from . import *
diff --git a/edg/core/test_connect_bundle.py b/edg/core/test_connect_bundle.py
new file mode 100644
index 000000000..a853629e4
--- /dev/null
+++ b/edg/core/test_connect_bundle.py
@@ -0,0 +1,26 @@
+import unittest
+
+from . import *
+from .HdlUserExceptions import UnconnectableError
+from .test_elaboration_common import TestBlockSource
+from .test_bundle import TestBundle
+
+
+class TestInner(Block):
+    def __init__(self) -> None:
+        super().__init__()
+        self.port = self.Port(TestBundle())
+
+
+class TestInvalidOuter(Block):
+    def __init__(self) -> None:
+        super().__init__()
+        self.inner = self.Block(TestInner())
+        self.source = self.Block(TestBlockSource())
+        self.connect(self.inner.port.a, self.source.source)
+
+
+class BundleInnerConnectError(unittest.TestCase):
+    def test_connect_error(self) -> None:
+        with self.assertRaises(UnconnectableError):
+            TestInvalidOuter()._elaborated_def_to_proto()