diff --git a/lib/node-excel-export.d.ts b/lib/node-excel-export.d.ts
index 3e2d4b7..4ccc2a0 100644
--- a/lib/node-excel-export.d.ts
+++ b/lib/node-excel-export.d.ts
@@ -20,10 +20,10 @@ declare module 'node-excel-export' {
     },
   };
 
-  type Heading = [{
-    value: string;
+  type Heading<TRowData> = [{
+    value: keyof TRowData;
     style: CellStyle
-  }| string[]][]
+  }| keyof TRowData[]][]
 
   type Merges = {
     start: {
@@ -47,9 +47,9 @@ declare module 'node-excel-export' {
   }
 
   function buildExport<TRowData>(sheets: {
-    name?: string;
-    heading?: Heading;
-    merges?: Merges;
+    name: string;
+    heading: Heading<TRowData> | undefined;
+    merges: Merges | undefined;
     specification: Specification<TRowData>;
     data: TRowData[];
   }[]): Buffer;
diff --git a/package.json b/package.json
index 77b12f8..f5da7d1 100755
--- a/package.json
+++ b/package.json
@@ -22,5 +22,8 @@
   "homepage": "https://github.com/andreyan-andreev/node-excel-export#readme",
   "dependencies": {
     "xlsx-style": "^0.8.13"
+  },
+  "devDependencies": {
+    "@types/node": "^10.12.9"
   }
 }