diff --git a/bignumber.js b/bignumber.js
index ecf03bd..017265b 100644
--- a/bignumber.js
+++ b/bignumber.js
@@ -2765,7 +2765,8 @@ function clone(configObject) {
         fractionPart = arr[1],
         len = intPart.length;
 
-      if (g2) {
+      // A groupSize of 0 disables grouping, regardless of secondaryGroupSize.
+      if (g1 > 0 && g2) {
         i = g1;
         g1 = g2;
         g2 = i;
diff --git a/test/methods/toFormat.js b/test/methods/toFormat.js
index 90d5d0f..0776952 100644
--- a/test/methods/toFormat.js
+++ b/test/methods/toFormat.js
@@ -279,6 +279,18 @@ Test('toFormat', function () {
 
     restoreDefaultFormat();
 
+    // groupSize: 0 must disable grouping even when secondaryGroupSize is set,
+    // instead of falling back to grouping by secondaryGroupSize alone.
+
+    BigNumber.config({ FORMAT: { groupSize: 0, secondaryGroupSize: 2 }});
+
+    t('123456789', 123456789);
+    t('1234.56', '1234.56');
+    t('-9.91', '-9.91');
+    t('1000037.123', '1000037.123456789', 3);
+
+    restoreDefaultFormat();
+
     //if (typeof window == 'undefined') {
     //    var vm = require('vm');
     //    t('1,234.57', '1234.567', vm.runInNewContext('[2, 2]'));