diff --git a/entrypoint.py b/entrypoint.py
index 463808d..5380753 100644
--- a/entrypoint.py
+++ b/entrypoint.py
@@ -110,6 +110,18 @@ def annotate_pr(message: str) -> None:
         if f.endswith(extension_filter)
     ]
 
+    if not files:
+        if extension_filter:
+            print(
+                f"Error: {case_path} is a directory but no files matched "
+                f"extension filter {extension_filter!r}."
+            )
+        else:
+            print(
+                f"Error: {case_path} is a directory but it contains no files to validate."
+            )
+        sys.exit(1)
+
     print(f"Validating {len(files)} files at: {case_path}")
     # Loop through each file and validate it
     has_failure: bool = False