diff --git a/pyeio/core/ext.py b/pyeio/core/ext.py
index 80e5275..d6bf98a 100644
--- a/pyeio/core/ext.py
+++ b/pyeio/core/ext.py
@@ -50,9 +50,12 @@ def standardize(extension: str) -> StandardExtension:
 
 def valid(
     extension: str,
-    allowed: set[str],
+    allowed: str | set[str],
     message: Optional[str] = None,
 ) -> None:
     """Validates expected extension."""
-    if extension != allowed:
-        raise Exception  # todo add custom
+    invalid = extension == allowed if isinstance(allowed, str) else extension in allowed
+    if invalid:
+        raise exc.InvalidExtensionError(
+            extension=extension, allowed=allowed, message=message
+        )