diff --git a/README.md b/README.md
index a60cf15..fbbd7a2 100644
--- a/README.md
+++ b/README.md
@@ -133,7 +133,7 @@ Examples of these are:
 ### Rule names
 Obtain the names of the currently implemented rules with:
 ```bash
- amarna --show-rules .
+ amarna --show-rules
 ```
 
 ### Rule allowlist
diff --git a/amarna/command_line.py b/amarna/command_line.py
index 8bf04a7..370a263 100644
--- a/amarna/command_line.py
+++ b/amarna/command_line.py
@@ -96,6 +96,7 @@ def main() -> int:
         metavar="-f",
         type=str,
         help="the name of the .cairo file or directory with .cairo files to analyze",
+        nargs="?",
     )
 
     parser.add_argument("-p", "--print", action="store_true", help="print output")
@@ -149,9 +150,17 @@ def main() -> int:
 
     filename = args.filename
 
+    if filename is None:
+        print("No file specified")
+        return -1
+
     if not os.path.isabs(filename):
         filename = os.path.join(os.getcwd(), filename)
 
+    if not os.path.exists(filename):
+        print(f"The specified file doesn't exist: {filename}")
+        return -1
+
     rule_set_names: List[str] = get_rule_names(args.rules, args.exclude_rules)
 
     results: List[Union[Result, ResultMultiplePositions]]