diff --git a/pkgs/source_maps/CHANGELOG.md b/pkgs/source_maps/CHANGELOG.md
index 7767fc1..c52d585 100644
--- a/pkgs/source_maps/CHANGELOG.md
+++ b/pkgs/source_maps/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.9.4
+
+* Update `SpanFormatException` with `source` and `offset`.
+
 ## 0.9.3
 
 * Support writing SingleMapping objects to source map version 3 format.
diff --git a/pkgs/source_maps/lib/span.dart b/pkgs/source_maps/lib/span.dart
index 771e9e0..b546a97 100644
--- a/pkgs/source_maps/lib/span.dart
+++ b/pkgs/source_maps/lib/span.dart
@@ -385,9 +385,10 @@
 
 /// A [SpanException] that's also a [FormatException].
 class SpanFormatException extends SpanException implements FormatException {
-  SpanFormatException(String message, Span span)
+  final source;
+
+  SpanFormatException(String message, Span span, [this.source])
       : super(message, span);
 
-  get source => null;
-  int get position => null;
+  int get offset => span == null ? null : span.start.offset;
 }
diff --git a/pkgs/source_maps/pubspec.yaml b/pkgs/source_maps/pubspec.yaml
index ae0323d..c5029e2 100644
--- a/pkgs/source_maps/pubspec.yaml
+++ b/pkgs/source_maps/pubspec.yaml
@@ -7,7 +7,7 @@
 #
 # When the minor version is upgraded, you *must* update that version constraint
 # in pub to stay in sync with this.
-version: 0.9.3
+version: 0.9.4
 author: Dart Team <misc@dartlang.org>
 description: Library to programmatically manipulate source map files.
 homepage: http://www.dartlang.org