diff --git a/src/lib/isFloat.js b/src/lib/isFloat.js
index 84bdc782c..5eeb9f511 100644
--- a/src/lib/isFloat.js
+++ b/src/lib/isFloat.js
@@ -5,7 +5,10 @@ import { decimal } from './alpha';
 export default function isFloat(str, options) {
   assertString(str);
   options = options || {};
-  const float = new RegExp(`^(?:[-+])?(?:[0-9]+)?(?:\\${options.locale ? decimal[options.locale] : '.'}[0-9]*)?(?:[eE][\\+\\-]?(?:[0-9]+))?$`);
+  // Fall back to `.` when the locale is unknown; otherwise `decimal[locale]`
+  // is `undefined` and stringifies into the pattern as the literal `undefined`.
+  const decimalSeparator = decimal[options.locale] || '.';
+  const float = new RegExp(`^(?:[-+])?(?:[0-9]+)?(?:\\${decimalSeparator}[0-9]*)?(?:[eE][\\+\\-]?(?:[0-9]+))?$`);
   if (str === '' || str === '.' || str === ',' || str === '-' || str === '+') {
     return false;
   }
diff --git a/test/validators.test.js b/test/validators.test.js
index 98d2a12ff..035611dc6 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -4857,6 +4857,23 @@ describe('Validators', () => {
         'foo',
       ],
     });
+    test({
+      validator: 'isFloat',
+      args: [{
+        locale: 'is-NOT-a-locale',
+      }],
+      valid: [
+        '123',
+        '123.123',
+        '-3.5e2',
+      ],
+      invalid: [
+        '3undefined5',
+        '123,123',
+        'foo',
+        '',
+      ],
+    });
     test({
       validator: 'isFloat',
       args: [{