diff --git a/lib/builder.dart b/lib/builder.dart
index 091e220..c8596fe 100644
--- a/lib/builder.dart
+++ b/lib/builder.dart
@@ -60,7 +60,7 @@
 }
 
 /// An entry in the source map builder.
-class Entry implements Comparable {
+class Entry implements Comparable<Entry> {
   /// Span denoting the original location in the input source file
   final SourceLocation source;