exceljs · guyonroche · Jan 17, 2020 · Jan 2, 2020 · Jan 2, 2020
diff --git a/lib/doc/workbook.js b/lib/doc/workbook.js
@@ -51,7 +51,7 @@ class Workbook {
       console.warn(`Worksheet name ${name} exceeds 31 chars. This will be truncated`);
     }
     name = (name || `sheet${id}`).substring(0, 31);
-    if (this._worksheets.find(ws => ws && ws.name === name)) {
+    if (this._worksheets.find(ws => ws && ws.name.toLowerCase() === name.toLowerCase())) {
       throw new Error(`Worksheet name already exists: ${name}`);
     }
 

diff --git a/spec/integration/worksheet.spec.js b/spec/integration/worksheet.spec.js
@@ -403,6 +403,18 @@ describe('Worksheet', () => {
     });
 
     context('when worksheet name already exists', () => {
+      it('throws an error', () => {
+        const wb = new ExcelJS.Workbook();
+
+        const validName = 'thisisaworksheetnameinuppercase';
+        const invalideName = 'THISISAWORKSHEETNAMEINUPPERCASE';
+        const expectedError = `Worksheet name already exists: ${invalideName}`;
+
+        wb.addWorksheet(validName);
+
+        expect(() => wb.addWorksheet(invalideName)).to.throw(expectedError);
+      });
+
       it('throws an error', () => {
         const wb = new ExcelJS.Workbook();