diff --git a/types/google-apps-script/google-apps-script.properties.d.ts b/types/google-apps-script/google-apps-script.properties.d.ts
index 184c46b28f..af38a01c9f 100644
--- a/types/google-apps-script/google-apps-script.properties.d.ts
+++ b/types/google-apps-script/google-apps-script.properties.d.ts
@@ -19,7 +19,7 @@ declare namespace GoogleAppsScript {
       deleteProperty(key: string): Properties;
       getKeys(): string[];
       getProperties(): Object;
-      getProperty(key: string): string;
+      getProperty(key: string): string | null;
       setProperties(properties: Object): Properties;
       setProperties(properties: Object, deleteAllOthers: boolean): Properties;
       setProperty(key: string, value: string): Properties;
@@ -56,7 +56,7 @@ declare namespace GoogleAppsScript {
       deleteProperty(key: string): ScriptProperties;
       getKeys(): string[];
       getProperties(): Object;
-      getProperty(key: string): string;
+      getProperty(key: string): string | null;
       setProperties(properties: Object): ScriptProperties;
       setProperties(properties: Object, deleteAllOthers: boolean): ScriptProperties;
       setProperty(key: string, value: string): ScriptProperties;
@@ -73,7 +73,7 @@ declare namespace GoogleAppsScript {
       deleteProperty(key: string): UserProperties;
       getKeys(): string[];
       getProperties(): Object;
-      getProperty(key: string): string;
+      getProperty(key: string): string | null;
       setProperties(properties: Object): UserProperties;
       setProperties(properties: Object, deleteAllOthers: boolean): UserProperties;
       setProperty(key: string, value: string): UserProperties;