diff --git a/tool/digest.cc b/tool/digest.cc
index 93058ed..224b79c 100644
--- a/tool/digest.cc
+++ b/tool/digest.cc
@@ -355,7 +355,7 @@
       break;
     }
 
-    if (arg[0] == "-") {
+    if (arg == "-") {
       // "-" ends the argument list and indicates that stdin should be used.
       sources.push_back(Source(Source::STDIN));
       break;