diff --git a/crc32.py b/crc32.py
index a51b761..d469409 100644
--- a/crc32.py
+++ b/crc32.py
@@ -1,4 +1,5 @@
 import argparse
+import os
 import sys
 import zlib
 
@@ -8,9 +9,9 @@ from voussoirkit import winglob
 def crc32_argparse(args):
     files = (
         file
-        for arg in args.source
-        for pattern in pipeable.input(arg)
+        for pattern in pipeable.input_many(args.patterns)
         for file in winglob.glob(pattern)
+        if os.path.isfile(file)
     )
     for file in files:
         try:
@@ -23,7 +24,7 @@ def crc32_argparse(args):
 def main(argv):
     parser = argparse.ArgumentParser(description=__doc__)
 
-    parser.add_argument('source', nargs='+')
+    parser.add_argument('patterns', nargs='+')
     parser.set_defaults(func=crc32_argparse)
 
     args = parser.parse_args(argv)