diff --git a/pkgs/source_span/CHANGELOG.md b/pkgs/source_span/CHANGELOG.md
index 5f87aee..10d94f6 100644
--- a/pkgs/source_span/CHANGELOG.md
+++ b/pkgs/source_span/CHANGELOG.md
@@ -1,3 +1,8 @@
+# 1.1.5
+
+* Fixed another case in which `FileSpan.union` could throw an exception for
+  external implementations of `FileSpan`.
+
 # 1.1.4
 
 * Eliminated dart2js warning about overriding `==`, but not `hashCode`.
diff --git a/pkgs/source_span/lib/src/file.dart b/pkgs/source_span/lib/src/file.dart
index aee3d78..e5bc53c 100644
--- a/pkgs/source_span/lib/src/file.dart
+++ b/pkgs/source_span/lib/src/file.dart
@@ -242,12 +242,17 @@
   SourceSpan union(SourceSpan other) {
     if (other is! FileSpan) return super.union(other);
 
+    
     _FileSpan span = expand(other);
-    var beginSpan = span._start == _start ? this : other;
-    var endSpan = span._end == _end ? this : other;
 
-    if (beginSpan._end < endSpan._start) {
-      throw new ArgumentError("Spans $this and $other are disjoint.");
+    if (other is _FileSpan) {
+      if (this._start > other._end || other._start > this._end) {
+        throw new ArgumentError("Spans $this and $other are disjoint.");
+      }
+    } else {
+      if (this._start > other.end.offset || other.start.offset > this._end) {
+        throw new ArgumentError("Spans $this and $other are disjoint.");
+      }
     }
 
     return span;
diff --git a/pkgs/source_span/pubspec.yaml b/pkgs/source_span/pubspec.yaml
index 3da9c28..2bfbe4e 100644
--- a/pkgs/source_span/pubspec.yaml
+++ b/pkgs/source_span/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_span
-version: 1.1.4
+version: 1.1.5
 author: Dart Team <misc@dartlang.org>
 description: A library for identifying source spans and locations.
 homepage: https://github.com/dart-lang/source_span