diff --git a/.changeset/dry-rabbits-grow.md b/.changeset/dry-rabbits-grow.md
new file mode 100644
index 000000000..4c0b204fb
--- /dev/null
+++ b/.changeset/dry-rabbits-grow.md
@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/css': patch
+---
+
+Fix runtime behavior to match the type definition
diff --git a/packages/css/src/style.ts b/packages/css/src/style.ts
index d2c417a1f..39a4bc2ea 100644
--- a/packages/css/src/style.ts
+++ b/packages/css/src/style.ts
@@ -28,10 +28,10 @@ function composedStyle(rules: Array<StyleRule | ClassNames>, debugId?: string) {
   const styleRules = [];
 
   for (const rule of rules) {
-    if (typeof rule === 'string') {
-      classList.push(rule);
-    } else {
+    if (typeof rule !== 'string' && !Array.isArray(rule)) {
       styleRules.push(rule);
+    } else {
+      classList.push(rule);
     }
   }