diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 86686cb9f6c..ac5ef01f865 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -71,7 +71,7 @@
         "category": "Error",
         "code": 1022
     },
-    "An index signature parameter type must be 'string' or 'number'.": {
+    "An index signature parameter type must be either 'string' or 'number'.": {
         "category": "Error",
         "code": 1023
     },