diff --git a/includes/Rest/Handler/Helper/HtmlInputTransformHelper.php b/includes/Rest/Handler/Helper/HtmlInputTransformHelper.php
index abe1fe0178a..7378b41369f 100644
--- a/includes/Rest/Handler/Helper/HtmlInputTransformHelper.php
+++ b/includes/Rest/Handler/Helper/HtmlInputTransformHelper.php
@@ -368,7 +368,14 @@ class HtmlInputTransformHelper {
 					throw new LocalizedHttpException( new MessageValue( "rest-bad-etag", [ $key ] ), 400 );
 				}
 			} else {
-				$originalRendering = ParsoidRenderID::newFromKey( $key );
+				try {
+					$originalRendering = ParsoidRenderID::newFromKey( $key );
+				} catch ( InvalidArgumentException $e ) {
+					throw new LocalizedHttpException(
+						new MessageValue( 'rest-parsoid-bad-render-id', [ $key ] ),
+						400
+					);
+				}
 			}
 		} elseif ( !empty( $original['html'] ) || !empty( $original['data-parsoid'] ) ) {
 			// NOTE: We might have an incomplete PageBundle here, with no HTML but with data-parsoid!
diff --git a/includes/Rest/i18n/en.json b/includes/Rest/i18n/en.json
index ebecc4fd98a..9555efa630d 100644
--- a/includes/Rest/i18n/en.json
+++ b/includes/Rest/i18n/en.json
@@ -65,6 +65,7 @@
 	"rest-unsupported-target-format": "The requested target format is not supported.",
 	"rest-parsoid-resource-exceeded": "Resource limit exceeded",
 	"rest-parsoid-error": "Parsoid error.",
+	"rest-parsoid-bad-render-id": "Bad Parsoid render ID: $1",
 	"rest-bad-stash-key": "Bad stash key.",
 	"rest-html-key-expected": "Expected <var>html</var> key in body",
 	"rest-invalid-transform": "Invalid transform: $1 to $2",
diff --git a/includes/Rest/i18n/qqq.json b/includes/Rest/i18n/qqq.json
index bd7a33191e8..32eca9e94cb 100644
--- a/includes/Rest/i18n/qqq.json
+++ b/includes/Rest/i18n/qqq.json
@@ -69,6 +69,7 @@
 	"rest-transform-missing-title": "Error message for REST API debugging, shown when no title or wikitext is provided",
 	"rest-unsupported-target-format": "Error message for REST API debugging, shown when the requested target format is not supported",
 	"rest-parsoid-resource-exceeded": "Error message for REST API debugging, shown when a resource limit is exceeded while converting between wikitext and HTML.",
+	"rest-parsoid-bad-render-id": "Error message for REST API debugging, shown when a ParsoidRenderID object cannot be created from the provided key. Parameters:\n* $1: The key",
 	"rest-parsoid-error": "Error message for REST API debugging, indicating an unspecified backend error occurred while converting between wikitext and HTML.\n\n[[:mw:Parsoid|Parsoid]] is the name of a software library; do not translate that name.",
 	"rest-bad-stash-key": "Error message for REST API debugging, shown when When the rednerid or etag given in the request is not a valid stash key.",
 	"rest-html-key-expected": "Error message for REST API debugging, shown when when the \"html\" key is missing from the request body",
diff --git a/tests/phpunit/integration/includes/Rest/Handler/Helper/HtmlInputTransformHelperTest.php b/tests/phpunit/integration/includes/Rest/Handler/Helper/HtmlInputTransformHelperTest.php
index 556657cb11d..e960f5d3a39 100644
--- a/tests/phpunit/integration/includes/Rest/Handler/Helper/HtmlInputTransformHelperTest.php
+++ b/tests/phpunit/integration/includes/Rest/Handler/Helper/HtmlInputTransformHelperTest.php
@@ -1188,6 +1188,20 @@ class HtmlInputTransformHelperTest extends MediaWikiIntegrationTestCase {
 		$helper->getContent();
 	}
 
+	public function testHandlesInvalidRenderID(): void {
+		$page = $this->getExistingTestPage( __METHOD__ );
+
+		$body = [ 'html' => 'hi', 'original' => [ 'renderid' => 'foo' ] ];
+		$params = [];
+
+		$this->expectExceptionObject( new LocalizedHttpException(
+			new MessageValue( 'rest-parsoid-bad-render-id', [ 'foo' ] ),
+			400
+		) );
+
+		$this->newHelper( [], StatsFactory::newNull(), $page, $body, $params );
+	}
+
 	private function newHtmlToContentTransform( $html, $methodOverrides = [] ): HtmlToContentTransform {
 		$transform = $this->getMockBuilder( HtmlToContentTransform::class )
 			->onlyMethods( array_keys( $methodOverrides ) )