diff --git a/src/string_util.cpp b/src/string_util.cpp
index 7f55f94..27a9840 100644
--- a/src/string_util.cpp
+++ b/src/string_util.cpp
@@ -17,6 +17,7 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  */
+#include <cerrno>
 #include <climits>
 #include <cstdio>
 #include <cstdlib>
@@ -24,6 +25,7 @@
 #include <syslog.h>
 #include <ctime>
 
+#include <stdexcept>
 #include <sstream>
 #include <string>
 #include <map>
@@ -50,45 +52,18 @@ template std::string str(unsigned long long value);
 
 static const char hexAlphabet[] = "0123456789ABCDEF";
 
+// replacement for C++11 std::stoll
 off_t s3fs_strtoofft(const char* str, bool is_base_16)
 {
-  if(!str || '\0' == *str){
-    return 0;
+  errno = 0;
+  char *temp;
+  long long result = strtoll(str, &temp, is_base_16 ? 16 : 10);
+
+  if(temp == str || *temp != '\0'){
+    throw std::invalid_argument("s3fs_strtoofft");
   }
-  off_t  result;
-  bool   chk_space;
-  bool   chk_base16_prefix;
-  for(result = 0, chk_space = false, chk_base16_prefix = false; '\0' != *str; str++){
-    // check head space
-    if(!chk_space && isspace(*str)){
-      continue;
-    }else if(!chk_space){
-      chk_space = true;
-    }
-    // check prefix for base 16
-    if(!chk_base16_prefix){
-      chk_base16_prefix = true;
-      if('0' == *str && ('x' == str[1] || 'X' == str[1])){
-        is_base_16 = true;
-        str++;
-        continue;
-      }
-    }
-    // check like isalnum and set data
-    result *= (is_base_16 ? 16 : 10);
-    if('0' <= *str && '9' >= *str){
-      result += static_cast<off_t>(*str - '0');
-    }else if(is_base_16){
-      if('A' <= *str && *str <= 'F'){
-        result += static_cast<off_t>(*str - 'A' + 0x0a);
-      }else if('a' <= *str && *str <= 'f'){
-        result += static_cast<off_t>(*str - 'a' + 0x0a);
-      }else{
-        return 0;
-      }
-    }else{
-      return 0;
-    }
+  if((result == LLONG_MIN || result == LLONG_MAX) && errno == ERANGE){
+    throw std::out_of_range("s3fs_strtoofft");
   }
   return result;
 }
diff --git a/src/string_util.h b/src/string_util.h
index 5a7bdb2..9b5e839 100644
--- a/src/string_util.h
+++ b/src/string_util.h
@@ -35,7 +35,7 @@ static inline int STR2NCMP(const char *str1, const char *str2) { return strncmp(
 
 template <class T> std::string str(T value);
 
-// Convert string to off_t.  Does not signal invalid input.
+// Convert string to off_t.  Throws std::invalid_argument and std::out_of_range on bad input.
 off_t s3fs_strtoofft(const char* str, bool is_base_16 = false);
 
 std::string trim_left(const std::string &s, const std::string &t = SPACES);
diff --git a/src/test_string_util.cpp b/src/test_string_util.cpp
index 82bcb0a..9cafc51 100644
--- a/src/test_string_util.cpp
+++ b/src/test_string_util.cpp
@@ -79,7 +79,12 @@ void test_strtoofft()
 {
   ASSERT_EQUALS(s3fs_strtoofft("0"), static_cast<off_t>(0L));
   ASSERT_EQUALS(s3fs_strtoofft("9"), static_cast<off_t>(9L));
-  ASSERT_EQUALS(s3fs_strtoofft("A"), static_cast<off_t>(0L));
+  try{
+    s3fs_strtoofft("A");
+    abort();
+  }catch(std::exception &e){
+    // expected
+  }
   ASSERT_EQUALS(s3fs_strtoofft("A", /*is_base_16=*/ true), static_cast<off_t>(10L));
   ASSERT_EQUALS(s3fs_strtoofft("F", /*is_base_16=*/ true), static_cast<off_t>(15L));
   ASSERT_EQUALS(s3fs_strtoofft("a", /*is_base_16=*/ true), static_cast<off_t>(10L));