diff --git a/src/lib/isFloat.js b/src/lib/isFloat.js
index 84bdc782c..3856d70a1 100644
--- a/src/lib/isFloat.js
+++ b/src/lib/isFloat.js
@@ -5,7 +5,14 @@ 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]+))?$`);
+  // Fall back to `.` unless the locale is an own key of `decimal`. Indexing
+  // `decimal[options.locale]` directly would also match inherited names such as
+  // `__proto__`, `constructor` or `toString`, whose truthy values leak into the
+  // pattern (e.g. the separator becomes the literal `[object Object]`).
+  const decimalSeparator = Object.prototype.hasOwnProperty.call(decimal, options.locale)
+    ? decimal[options.locale]
+    : '.';
+  const float = new RegExp(`^(?:[-+])?(?:[0-9]+)?(?:\\${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..b1f8f8400 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -4857,6 +4857,42 @@ describe('Validators', () => {
         'foo',
       ],
     });
+    test({
+      validator: 'isFloat',
+      args: [{
+        locale: 'is-NOT-a-locale',
+      }],
+      valid: [
+        '123',
+        '123.123',
+        '-3.5e2',
+      ],
+      invalid: [
+        '3undefined5',
+        '123,123',
+        'foo',
+        '',
+      ],
+    });
+    // Inherited property names (e.g. __proto__, constructor) are not own keys of
+    // the decimal map, so they must fall back to the "." separator too.
+    for (const inheritedLocale of ['__proto__', 'constructor', 'toString', 'hasOwnProperty', 'valueOf']) {
+      test({
+        validator: 'isFloat',
+        args: [{ locale: inheritedLocale }],
+        valid: [
+          '123',
+          '123.123',
+          '-3.5e2',
+        ],
+        invalid: [
+          '123,123',
+          '3[object Object]5',
+          'foo',
+          '',
+        ],
+      });
+    }
     test({
       validator: 'isFloat',
       args: [{