diff --git a/src/fstring.cpp b/src/fstring.cpp
index 4d06f2f3..405f28a4 100644
--- a/src/fstring.cpp
+++ b/src/fstring.cpp
@@ -139,7 +139,9 @@ FString::FString (const FString& s)  // copy constructor
   , bufsize(0)
   , c_string(0)
 {
-  if ( s.string )
+  if ( s.isNull() )
+    return;
+  else
     _assign (s.string);
 }
 
@@ -150,8 +152,10 @@ FString::FString (const std::wstring& s)
   , bufsize(0)
   , c_string(0)
 {
-  if ( ! s.empty() )
-    _assign ( s.c_str() );
+  if ( s.empty() )
+    _assign (L"");
+  else
+    _assign (s.c_str());
 }
 
 //----------------------------------------------------------------------
@@ -164,7 +168,12 @@ FString::FString (const wchar_t s[])
   if ( ! s )
     return;
 
-  if ( s[0] )
+  if ( ! s[0] )
+  {
+    _assign(L"");
+    return;
+  }
+  else
     _assign (s);
 }
 
@@ -176,7 +185,10 @@ FString::FString (const std::string& s)
   , c_string(0)
 {
   if ( s.empty() )
+  {
+    _assign(L"");
     return;
+  }
 
   const wchar_t* wc_string;
   wc_string = c_to_wc_str(s.c_str());
@@ -195,7 +207,10 @@ FString::FString (const char s[])
     return;
 
   if ( ! s[0] )
+  {
+    _assign(L"");
     return;
+  }
 
   const wchar_t* wc_string;
   wc_string = c_to_wc_str(s);
@@ -211,7 +226,10 @@ FString::FString (const wchar_t c)
   , c_string(0)
 {
   if ( c == 0 )
+  {
+    _assign(L"");
     return;
+  }
 
   wchar_t s[2];
   s[0] = c;
@@ -227,7 +245,10 @@ FString::FString (const char c)
   , c_string(0)
 {
   if ( c == 0 )
+  {
+    _assign(L"");
     return;
+  }
 
   wchar_t s[2];
   s[0] = wchar_t(c & 0xff);
@@ -250,11 +271,7 @@ FString::~FString()  // destructor
 //----------------------------------------------------------------------
 FString& FString::operator = (const FString& s)
 {
-  if ( s )
-    _assign (s.string);
-  else
-    clear();
-
+  _assign (s.string);
   return *this;
 }
 
@@ -1042,10 +1059,10 @@ float FString::toFloat() const
   register double num;
   num = toDouble();
 
-  if ( num > double(FLT_MAX) )
+  if ( num > double(FLT_MAX) || num < double(-FLT_MAX) )
     throw std::overflow_error ("overflow");
 
-  if ( num < double(-FLT_MAX) )
+  if ( std::fabs(num) < FLT_EPSILON )  // num == 0.0f
     throw std::underflow_error ("underflow");
 
   return float(num);
@@ -1116,7 +1133,7 @@ FString FString::rtrim() const
     s.length--;
 
   if ( last == p && std::iswspace(wint_t(*last)) )
-    s.clear();
+    s = L"";
   else
     *(last + 1) = '\0';
 
diff --git a/src/test/fstring-test.cpp b/src/test/fstring-test.cpp
index 589d0c62..b9afe6e0 100644
--- a/src/test/fstring-test.cpp
+++ b/src/test/fstring-test.cpp
@@ -145,14 +145,13 @@ void FStringTest::NoArgumentTest()
   CPPUNIT_ASSERT ( str.size() == 0 );
   CPPUNIT_ASSERT ( str.empty() );
   const FString fstr = str;
-  CPPUNIT_ASSERT ( fstr == empty );
-  CPPUNIT_ASSERT ( empty == '\0' );
-  CPPUNIT_ASSERT ( empty == L'\0' );
+  CPPUNIT_ASSERT ( ! fstr.isNull() );
+  CPPUNIT_ASSERT ( fstr.isEmpty() );
+
   cstr = 0;
   CPPUNIT_ASSERT ( empty == cstr );
   wcstr = 0;
-  CPPUNIT_ASSERT ( empty == std::string() );
-  CPPUNIT_ASSERT ( empty == std::wstring() );
+  CPPUNIT_ASSERT ( empty == wcstr );
 
   CPPUNIT_ASSERT ( ! empty.includes('A') );
   CPPUNIT_ASSERT ( ! empty.includes(L'A') );
@@ -822,14 +821,15 @@ void FStringTest::functionCallOperatorTest()
 //----------------------------------------------------------------------
 void FStringTest::formatTest()
 {
-  FString str;
+  FString str1;
   int num = 3;
   char location[] = "zoo";
-  str.sprintf ("There are %d lions in the %s", num, location);
-  CPPUNIT_ASSERT ( str == "There are 3 lions in the zoo" );
+  str1.sprintf ("There are %d lions in the %s", num, location);
+  CPPUNIT_ASSERT ( str1 == "There are 3 lions in the zoo" );
 
-  str.sprintf (L"It costs only %d cent", 50);
-  CPPUNIT_ASSERT ( str == "It costs only 50 cent" );
+  FString str2;
+  str2.sprintf (L"It costs only %d cent", 50);
+  CPPUNIT_ASSERT ( str2 == "It costs only 50 cent" );
 
   std::setlocale (LC_NUMERIC, "C");
   FString fnum1, fnum2;
@@ -869,9 +869,15 @@ void FStringTest::convertToNumberTest()
   str = "-2147483647";
   CPPUNIT_ASSERT ( str.toLong() == -2147483647 );
 
+  str = "+987654321";
+  CPPUNIT_ASSERT ( str.toLong() == 987654321 );
+
   str = "4294967295";
   CPPUNIT_ASSERT ( str.toULong() == 4294967295 );
 
+  str = "+1234567890";
+  CPPUNIT_ASSERT ( str.toULong() == 1234567890 );
+
   str = "3.14159";
   CPPUNIT_ASSERT ( str.toFloat() == 3.14159f );
 
@@ -950,6 +956,24 @@ void FStringTest::exceptionTest()
 
   CPPUNIT_ASSERT_THROW ( FString("one").toULong()
                        , std::invalid_argument );
+
+  CPPUNIT_ASSERT_THROW ( FString("1E+42").toFloat()
+                       , std::overflow_error );
+
+  CPPUNIT_ASSERT_THROW ( FString("-1E+42").toFloat()
+                       , std::overflow_error );
+
+  CPPUNIT_ASSERT_THROW ( FString("1.19209290E-08").toFloat()
+                       , std::underflow_error );
+
+  CPPUNIT_ASSERT_THROW ( FString("1.7976931348623157E+309").toDouble()
+                       , std::overflow_error );
+
+  CPPUNIT_ASSERT_THROW ( FString("-1.7976931348623157E+309").toDouble()
+                       , std::overflow_error );
+
+  CPPUNIT_ASSERT_THROW ( FString("2.225074e-310").toDouble()
+                       , std::underflow_error );
 }
 
 //----------------------------------------------------------------------
@@ -962,15 +986,15 @@ void FStringTest::trimTest()
 
   const FString& trim_str2 = L"\n  \n\n";
   CPPUNIT_ASSERT ( trim_str2.rtrim().isEmpty() );
-  CPPUNIT_ASSERT ( trim_str2.rtrim().isNull() );
+  CPPUNIT_ASSERT ( ! trim_str2.rtrim().isNull() );
   CPPUNIT_ASSERT ( trim_str2.rtrim().getLength() == 0 );
 
   CPPUNIT_ASSERT ( trim_str2.ltrim().isEmpty() );
-  CPPUNIT_ASSERT ( trim_str2.ltrim().isNull() );
+  CPPUNIT_ASSERT ( ! trim_str2.ltrim().isNull() );
   CPPUNIT_ASSERT ( trim_str2.ltrim().getLength() == 0 );
 
   CPPUNIT_ASSERT ( trim_str2.trim().isEmpty() );
-  CPPUNIT_ASSERT ( trim_str2.trim().isNull() );
+  CPPUNIT_ASSERT ( ! trim_str2.trim().isNull() );
   CPPUNIT_ASSERT ( trim_str2.trim().getLength() == 0 );
 }