diff --git a/substratools/utils.py b/substratools/utils.py
index 02cf5da..2f05b99 100644
--- a/substratools/utils.py
+++ b/substratools/utils.py
@@ -101,9 +101,17 @@ def load_interface_from_module(module_name, interface_class, interface_signature
         )
 
     # find interface class
+    found_interfaces = []
     for _, obj in inspect.getmembers(module, inspect.isclass):
-        if issubclass(obj, interface_class):
-            return obj()  # return interface instance
+        if issubclass(obj, interface_class) and obj != interface_class:
+            found_interfaces.append(obj)
+
+    if len(found_interfaces) == 1:
+        return found_interfaces[0]()  # return interface instance
+    elif len(found_interfaces) > 1:
+        raise exceptions.InvalidInterfaceError(
+            f"Multiple interfaces found in module '{module_name}': {found_interfaces}"
+        )
 
     # backward compatibility; accept methods at module level directly
     if interface_signature is None:
diff --git a/tests/test_opener.py b/tests/test_opener.py
index 58e8869..8c235d2 100644
--- a/tests/test_opener.py
+++ b/tests/test_opener.py
@@ -74,6 +74,36 @@ def test_load_opener_from_path(tmp_cwd, valid_opener_code):
     assert o.get_data()[0] == "X"
 
 
+def test_load_opener_from_path_error_with_inheritance(tmp_cwd):
+    wrong_opener_code = """
+import json
+from substratools import Opener
+
+class FakeOpener(Opener):
+    def get_data(self, folder):
+        return 'X', list(range(0, 3))
+
+    def fake_data(self, n_samples):
+        return ['Xfake'] * n_samples, [0] * n_samples
+
+class FinalOpener(FakeOpener):
+    def __init__(self):
+        super().__init__()
+"""
+    dirpath = tmp_cwd / "myopener"
+    dirpath.mkdir()
+    path = dirpath / "my_opener.py"
+    path.write_text(wrong_opener_code)
+
+    with pytest.raises(exceptions.InvalidInterfaceError):
+        load_interface_from_module(
+            "opener",
+            interface_class=Opener,
+            interface_signature=None,  # XXX does not support interface for debugging
+            path=path,
+        )
+
+
 def test_opener_check_folders(tmp_cwd):
     script = """
 from substratools import Opener