diff --git a/traitlets/config/configurable.py b/traitlets/config/configurable.py
index 44b4793e..9958ccdb 100644
--- a/traitlets/config/configurable.py
+++ b/traitlets/config/configurable.py
@@ -590,8 +590,8 @@ def instance(cls: type[CT], *args: t.Any, **kwargs: t.Any) -> CT:
             return cls._instance
         else:
             raise MultipleInstanceError(
-                f"An incompatible sibling of '{cls.__name__}' is already instantiated"
-                f" as singleton: {type(cls._instance).__name__}"
+                f"An incompatible sibling of '{cls.__module__}.{cls.__name__}' is already instantiated"
+                f" as singleton: {type(cls._instance).__module__}.{type(cls._instance).__name__}"
             )
 
     @classmethod