diff --git a/includes/ImageMap.php b/includes/ImageMap.php
index 6fc24eb..d67f20e 100644
--- a/includes/ImageMap.php
+++ b/includes/ImageMap.php
@@ -279,9 +279,11 @@ class ImageMap {
if ( $realmap ) {
// Construct the map
- // Add random number to avoid breaking cached HTML fragments that are
- // later joined together on the one page (bug 16471)
- $mapName = "ImageMap_" . ++self::$id . '_' . mt_rand( 0, 0x7fffffff );
+ // Add a hash of the map HTML to avoid breaking cached HTML fragments that are
+ // later joined together on the one page (T18471).
+ // The only way these hashes can clash is if the map is identical, in which
+ // case it wouldn't matter that the "wrong" map was used.
+ $mapName = 'ImageMap_' . substr( md5( $mapHTML ), 0, 16 );
$mapHTML = "\n";
// Alter the image tag
diff --git a/tests/parser/imageMapParserTests.txt b/tests/parser/imageMapParserTests.txt
index 70357ef..ef677e1 100644
--- a/tests/parser/imageMapParserTests.txt
+++ b/tests/parser/imageMapParserTests.txt
@@ -7,3 +7,15 @@ Image:Foobar.jpg
!! result
!! end
+
+!! test
+dummy test
+!! input
+
+File:Foobar.jpg
+
+poly 10.1 10.9 10 30 -30 15 [[Main Page]]
+
+!! result
+
+!! end
