diff --git a/news/418.bugfix b/news/418.bugfix
new file mode 100644
index 000000000..b846ed1da
--- /dev/null
+++ b/news/418.bugfix
@@ -0,0 +1 @@
+OmegaConf.to_container() raises a ValueError on invalid input
diff --git a/omegaconf/omegaconf.py b/omegaconf/omegaconf.py
index c085a6617..6eb69939f 100644
--- a/omegaconf/omegaconf.py
+++ b/omegaconf/omegaconf.py
@@ -514,8 +514,11 @@ def to_container(
                (DictConfigs backed by a dataclass)
         :return: A dict or a list representing this config as a primitive container.
         """
-        assert isinstance(cfg, Container)
-        # noinspection PyProtectedMember
+        if not OmegaConf.is_config(cfg):
+            raise ValueError(
+                f"Input cfg is not an OmegaConf config object ({type_str(type(cfg))})"
+            )
+
         return BaseContainer._to_content(
             cfg,
             resolve=resolve,
diff --git a/tests/test_base_config.py b/tests/test_base_config.py
index 8638b8bfa..d8fa32c59 100644
--- a/tests/test_base_config.py
+++ b/tests/test_base_config.py
@@ -1,4 +1,5 @@
 import copy
+import re
 from enum import Enum
 from typing import Any, Dict, List, Union
 
@@ -210,6 +211,14 @@ def test_to_container(src: Any, expected: Any, expected_with_resolve: Any) -> No
     assert container == expected_with_resolve
 
 
+def test_to_container_invalid_input() -> None:
+    with pytest.raises(
+        ValueError,
+        match=re.escape("Input cfg is not an OmegaConf config object (dict)"),
+    ):
+        OmegaConf.to_container({})
+
+
 def test_string_interpolation_with_readonly_parent() -> None:
     cfg = OmegaConf.create({"a": 10, "b": {"c": "hello_${a}"}})
     OmegaConf.set_readonly(cfg, True)