diff --git a/lib/helpers.js b/lib/helpers.js
index 508432f..a83b02b 100644
--- a/lib/helpers.js
+++ b/lib/helpers.js
@@ -186,6 +186,10 @@ FORMAT_REGEXPS.pattern = FORMAT_REGEXPS.regex;
 FORMAT_REGEXPS.ipv4 = FORMAT_REGEXPS['ip-address'];
 
 exports.isFormat = function isFormat (input, format, validator) {
+  if (validator && validator.customFormats &&
+    typeof validator.customFormats[format] === 'function') {
+    return validator.customFormats[format](input);
+  }
   if (typeof input === 'string' && FORMAT_REGEXPS[format] !== undefined) {
     if (FORMAT_REGEXPS[format] instanceof RegExp) {
       return FORMAT_REGEXPS[format].test(input);
@@ -193,9 +197,6 @@ exports.isFormat = function isFormat (input, format, validator) {
     if (typeof FORMAT_REGEXPS[format] === 'function') {
       return FORMAT_REGEXPS[format](input);
     }
-  } else if (validator && validator.customFormats &&
-      typeof validator.customFormats[format] === 'function') {
-    return validator.customFormats[format](input);
   }
   return true;
 };
diff --git a/test/formats.js b/test/formats.js
index 830465a..7db6c36 100644
--- a/test/formats.js
+++ b/test/formats.js
@@ -328,6 +328,16 @@ describe('Formats', function () {
         ((new Validator()).customFormats.boo).should.be.a('function');
       });
     });
+
+    describe('override default format validation', function () {
+      beforeEach(function () {
+        this.validator.customFormats.date = input => false
+      })
+
+      it('should fail any date validation', function () {
+        this.validator.validate('2021-02-01', {type: 'string', format: 'date'}).valid.should.be.false
+      })
+    })
   });
 
   describe('with options.disableFormat === true', function() {