diff --git a/src/Symfony/Component/Translation/MessageCatalogue.php b/src/Symfony/Component/Translation/MessageCatalogue.php
index 2bad9c2c2f8b..cde056a73609 100644
--- a/src/Symfony/Component/Translation/MessageCatalogue.php
+++ b/src/Symfony/Component/Translation/MessageCatalogue.php
@@ -158,9 +158,17 @@ public function replace($messages, $domain = 'messages')
     public function add($messages, $domain = 'messages')
     {
         if (!isset($this->messages[$domain])) {
-            $this->messages[$domain] = $messages;
-        } else {
-            foreach ($messages as $id => $message) {
+            $this->messages[$domain] = [];
+        }
+        $intlDomain = $domain;
+        $suffixLength = \strlen(self::INTL_DOMAIN_SUFFIX);
+        if (\strlen($domain) > $suffixLength && false !== strpos($domain, self::INTL_DOMAIN_SUFFIX, -$suffixLength)) {
+            $intlDomain .= self::INTL_DOMAIN_SUFFIX;
+        }
+        foreach ($messages as $id => $message) {
+            if (isset($this->messages[$intlDomain]) && \array_key_exists($id, $this->messages[$intlDomain])) {
+                $this->messages[$intlDomain][$id] = $message;
+            } else {
                 $this->messages[$domain][$id] = $message;
             }
         }
diff --git a/src/Symfony/Component/Translation/Tests/Catalogue/TargetOperationTest.php b/src/Symfony/Component/Translation/Tests/Catalogue/TargetOperationTest.php
index 570b503aeaba..d4608f6ef631 100644
--- a/src/Symfony/Component/Translation/Tests/Catalogue/TargetOperationTest.php
+++ b/src/Symfony/Component/Translation/Tests/Catalogue/TargetOperationTest.php
@@ -67,6 +67,19 @@ public function testGetResultFromIntlDomain()
         );
     }
 
+    public function testGetResultWithMixedDomains()
+    {
+        $this->assertEquals(
+            new MessageCatalogue('en', [
+                'messages+intl-icu' => ['a' => 'old_a'],
+            ]),
+            $this->createOperation(
+                new MessageCatalogue('en', ['messages' => ['a' => 'old_a']]),
+                new MessageCatalogue('en', ['messages+intl-icu' => ['a' => 'new_a']])
+            )->getResult()
+        );
+    }
+
     public function testGetResultWithMetadata()
     {
         $leftCatalogue = new MessageCatalogue('en', ['messages' => ['a' => 'old_a', 'b' => 'old_b']]);