diff --git a/src/swab.rs b/src/swab.rs
index 7889a59..5b4c173 100644
--- a/src/swab.rs
+++ b/src/swab.rs
@@ -48,7 +48,8 @@ fn main() -> ExitCode {
 	let mut input = stdin();
 	let mut output = stdout().lock();
 
-	let mut wordsize: usize = 2;
+	let mut optind: usize = 1;   // argv[0]
+	let mut wordsize: usize = 2; // Equivalent to dd(1p).
 
 	while let Some(opt) = argv.getopt(":w:") {
 		match opt.opt() {
@@ -59,11 +60,16 @@ fn main() -> ExitCode {
 						_ => { return usage(&argv[0]); },
 					}
 				}
+				optind = opt.ind();
 			},
 			_ => { return usage(&argv[0]); }
 		}
 	}
 
+	if optind < argv.len() {
+		return usage(&argv[0]);
+	}
+
 	buf.resize(wordsize, 0);
 
 	loop {