diff --git a/lib/Page.js b/lib/Page.js
index 6b9617e8..1fb0c89b 100644
--- a/lib/Page.js
+++ b/lib/Page.js
@@ -470,7 +470,7 @@ class Page extends EventEmitter {
     let paperWidth = 8.5;
     let paperHeight = 11;
     if (options.format) {
-      let format = Page.PaperFormats[options.format];
+      let format = Page.PaperFormats[options.format.toLowerCase()];
       console.assert(format, 'Unknown paper format: ' + options.format);
       paperWidth = format.width;
       paperHeight = format.height;
@@ -627,16 +627,16 @@ class Page extends EventEmitter {
 
 /** @enum {string} */
 Page.PaperFormats = {
-  Letter: {width: 8.5, height: 11},
-  Legal: {width: 8.5, height: 14},
-  Tabloid: {width: 11, height: 17},
-  Ledger: {width: 17, height: 11},
-  A0: {width: 33.1, height: 46.8 },
-  A1: {width: 23.4, height: 33.1 },
-  A2: {width: 16.5, height: 23.4 },
-  A3: {width: 11.7, height: 16.5 },
-  A4: {width: 8.27, height: 11.7 },
-  A5: {width: 5.83, height: 8.27 },
+  letter: {width: 8.5, height: 11},
+  legal: {width: 8.5, height: 14},
+  tabloid: {width: 11, height: 17},
+  ledger: {width: 17, height: 11},
+  a0: {width: 33.1, height: 46.8 },
+  a1: {width: 23.4, height: 33.1 },
+  a2: {width: 16.5, height: 23.4 },
+  a3: {width: 11.7, height: 16.5 },
+  a4: {width: 8.27, height: 11.7 },
+  a5: {width: 5.83, height: 8.27 },
 };
 
 let unitToPixels = {
diff --git a/test/test.js b/test/test.js
index b03b91df..21591947 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1407,7 +1407,7 @@ describe('Page', function() {
     }));
     it('should support setting custom format', SX(async function() {
       let pages = await getPDFPages(await page.pdf({
-        format: 'A4'
+        format: 'a4'
       }));
       expect(pages.length).toBe(1);
       expect(pages[0].width).toBeCloseTo(8.27, 1);