diff --git a/lib/src/io.dart b/lib/src/io.dart
index 20792c13d30bf53d08d9bbb79cd0f554aa43856e..1620837c2b2f3362997abbf78ac581a52b543dab 100644
--- a/lib/src/io.dart
+++ b/lib/src/io.dart
@@ -255,7 +255,7 @@ void createPackageSymlink(String name, String target, String symlink,
 
 /// Whether pub is running from within the Dart SDK, as opposed to from the Dart
 /// source repository.
-bool get runningFromSdk => path.extension(new Options().script) == 'snapshot';
+bool get runningFromSdk => path.extension(new Options().script) == '.snapshot';
 
 /// Resolves [target] relative to the path to pub's `resource` directory.
 String resourcePath(String target) {