Negative value can be returned from function is not being checked before being used improperly

Defect 112121 (STATIC_C)
Checker NEGATIVE_RETURNS (subcategory none)
File: /src/mongo/db/pipeline/expression.cpp
Function mongo::Expression::removeFieldPrefix(const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> &)::[lambda() (instance 1)]::operator ()() const
/src/mongo/db/pipeline/expression.cpp, line: 77
Function "this->prefixedField->find("", 0UL)" returns a negative number. [Note: The source code implementation of the function has been overridden by a builtin model.]

        uassert(16419,

/src/mongo/db/pipeline/expression.cpp, line: 77
Assigning: "<temporary>" = "this->prefixedField->find("", 0UL)".

        uassert(16419,