diff --git a/dev-infra/format/config.ts b/dev-infra/format/config.ts
index e6713aabe9..62be7a6ca9 100644
--- a/dev-infra/format/config.ts
+++ b/dev-infra/format/config.ts
@@ -13,7 +13,7 @@ interface Formatter {
 }
 
 export interface FormatConfig {
-  [keyof: string]: boolean|Formatter;
+  [key: string]: boolean|Formatter;
 }
 
 /** Retrieve and validate the config as `FormatConfig`. */