diff --git a/src/Symfony/Component/Yaml/Parser.php b/src/Symfony/Component/Yaml/Parser.php
index 42f61af9de..4a5f2b91e0 100644
--- a/src/Symfony/Component/Yaml/Parser.php
+++ b/src/Symfony/Component/Yaml/Parser.php
@@ -237,7 +237,8 @@ class Parser
                 }
 
                 if (!(Yaml::PARSE_KEYS_AS_STRINGS & $flags) && !is_string($key)) {
-                    @trigger_error('Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.', E_USER_DEPRECATED);
+                    $keyType = is_numeric($key) ? 'numeric key' : 'incompatible key type';
+                    @trigger_error(sprintf('Implicit casting of %s to string on line %d is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.', $keyType, $this->getRealCurrentLineNb()), E_USER_DEPRECATED);
                 }
 
                 // Convert float keys to strings, to avoid being converted to integers by PHP
diff --git a/src/Symfony/Component/Yaml/Tests/ParserTest.php b/src/Symfony/Component/Yaml/Tests/ParserTest.php
index a0bef9d528..a6c7b6ad12 100644
--- a/src/Symfony/Component/Yaml/Tests/ParserTest.php
+++ b/src/Symfony/Component/Yaml/Tests/ParserTest.php
@@ -1081,7 +1081,7 @@ EOF;
 
     /**
      * @group legacy
-     * @expectedDeprecation Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
+     * @expectedDeprecation Implicit casting of numeric key to string on line 1 is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
      */
     public function testFloatKeys()
     {
@@ -1103,7 +1103,7 @@ EOF;
 
     /**
      * @group legacy
-     * @expectedDeprecation Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
+     * @expectedDeprecation Implicit casting of incompatible key type to string on line 1 is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
      */
     public function testBooleanKeys()
     {