diff --git a/include/boost/multiprecision/cpp_float.hpp b/include/boost/multiprecision/cpp_float.hpp
index 41ad3005..7a33dfa9 100644
--- a/include/boost/multiprecision/cpp_float.hpp
+++ b/include/boost/multiprecision/cpp_float.hpp
@@ -1755,13 +1755,13 @@ std::string cpp_float<Digits10>::str(std::streamsize number_of_digits, std::ios_
       str.append(1, 'e');
       str.append(boost::lexical_cast<std::string>(my_exp));
    }
-   if(showpoint || scientific)
+   if((showpoint || scientific) && (number_of_digits != (std::numeric_limits<std::size_t>::max)()))
    {
       std::streamsize chars = str.size() - 1;
       BOOST_ASSERT(str.find('.') != std::string::npos); // there must be a decimal point!!
       chars = number_of_digits - chars;
       if(chars > 0)
-         str.append(chars, '0');
+         str.append(static_cast<std::string::size_type>(chars), '0');
    }
    if(isneg())
       str.insert(0, 1, '-');