diff --git a/src/utils/utilities.ts b/src/utils/utilities.ts
index 9e8929c..b7dbe89 100644
--- a/src/utils/utilities.ts
+++ b/src/utils/utilities.ts
@@ -23,7 +23,7 @@ export function directionSize(propertyPrefix: string): DynamicMatcher {
     if (v != null) {
       return directionMap[direction].map(i => [`${propertyPrefix}${i}`, v])
     }
-    else if (size.startsWith('-')) {
+    else if (size?.startsWith('-')) {
       // --custom-spacing-value
       const v = theme.spacing?.[size.slice(1)]
       if (v != null)