diff --git a/Tests/LibRegex/Regex.cpp b/Tests/LibRegex/Regex.cpp
index 6b8f5f559c..2b11be4d37 100644
--- a/Tests/LibRegex/Regex.cpp
+++ b/Tests/LibRegex/Regex.cpp
@@ -599,6 +599,7 @@ TEST_CASE(ECMA262_parse)
         { "(?<$$_$$>a)"sv },
         { "(?<ÿ>a)"sv },
         { "(?<𝓑𝓻𝓸𝔀𝓷>a)"sv },
+        { "((?=lg)?[vl]k\\-?\\d{3}) bui| 3\\.[-\\w; ]{10}lg?-([06cv9]{3,4})"sv, regex::Error::NoError, ECMAScriptFlags::BrowserExtended } // #12373, quantifiable assertions.
     };
 
     for (auto& test : tests) {
diff --git a/Userland/Libraries/LibRegex/RegexParser.cpp b/Userland/Libraries/LibRegex/RegexParser.cpp
index 564f57e612..60d960a655 100644
--- a/Userland/Libraries/LibRegex/RegexParser.cpp
+++ b/Userland/Libraries/LibRegex/RegexParser.cpp
@@ -1083,6 +1083,9 @@ bool ECMA262Parser::parse_assertion(ByteCode& stack, [[maybe_unused]] size_t& ma
         if (m_should_use_browser_extended_grammar) {
             if (!unicode) {
                 if (parse_quantifiable_assertion(assertion_stack, match_length_minimum, named)) {
+                    if (!parse_quantifier(assertion_stack, match_length_minimum, unicode, named))
+                        return false;
+
                     stack.extend(move(assertion_stack));
                     return true;
                 }