diff --git a/AK/Utf16View.cpp b/AK/Utf16View.cpp
index 6f0f4677714..de6ef1d546f 100644
--- a/AK/Utf16View.cpp
+++ b/AK/Utf16View.cpp
@@ -185,6 +185,8 @@ bool Utf16View::validate(size_t& valid_code_units, AllowLonelySurrogates allow_l
 
 size_t Utf16View::code_unit_offset_of(size_t code_point_offset) const
 {
+    VERIFY(code_point_offset <= length_in_code_points());
+
     if (length_in_code_points() == length_in_code_units()) // Fast path: all code points are one code unit.
         return code_point_offset;
 
@@ -203,6 +205,8 @@ size_t Utf16View::code_unit_offset_of(size_t code_point_offset) const
 
 size_t Utf16View::code_point_offset_of(size_t code_unit_offset) const
 {
+    VERIFY(code_unit_offset <= length_in_code_units());
+
     if (length_in_code_points() == length_in_code_units()) // Fast path: all code points are one code unit.
         return code_unit_offset;
 
diff --git a/Libraries/LibJS/Runtime/RegExpPrototype.cpp b/Libraries/LibJS/Runtime/RegExpPrototype.cpp
index 7d765c76032..ceeccfe2050 100644
--- a/Libraries/LibJS/Runtime/RegExpPrototype.cpp
+++ b/Libraries/LibJS/Runtime/RegExpPrototype.cpp
@@ -221,7 +221,10 @@ static ThrowCompletionOr<Value> regexp_builtin_exec(VM& vm, RegExpObject& regexp
     //       ii. Set matchSucceeded to true.
 
     // 13.b and 13.c
-    regex.start_offset = full_unicode ? string->utf16_string_view().code_point_offset_of(last_index) : last_index;
+    regex.start_offset = full_unicode && last_index <= string->length_in_utf16_code_units()
+        ? string->utf16_string_view().code_point_offset_of(last_index)
+        : last_index;
+
     result = regex.match(string->utf16_string_view());
 
     // 13.d and 13.a