diff --git a/.changeset/lucky-comics-bow.md b/.changeset/lucky-comics-bow.md
new file mode 100644
index 000000000..92af2ec29
--- /dev/null
+++ b/.changeset/lucky-comics-bow.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+check if class:list's value is defined before converting
diff --git a/packages/astro/src/runtime/server/hydration.ts b/packages/astro/src/runtime/server/hydration.ts
index 436e19f3c..cb26b72e8 100644
--- a/packages/astro/src/runtime/server/hydration.ts
+++ b/packages/astro/src/runtime/server/hydration.ts
@@ -92,8 +92,10 @@ export function extractDirectives(inputProps: Record<string | number, any>): Ext
 				}
 			}
 		} else if (key === 'class:list') {
-			// support "class" from an expression passed into a component (#782)
-			extracted.props[key.slice(0, -5)] = serializeListValue(value);
+			if (value) {
+				// support "class" from an expression passed into a component (#782)
+				extracted.props[key.slice(0, -5)] = serializeListValue(value);
+			}
 		} else {
 			extracted.props[key] = value;
 		}