diff --git a/reactos/lib/sdk/crt/string/atoi.c b/reactos/lib/sdk/crt/string/atoi.c
index 74f3175565e..2fc1364dc83 100644
--- a/reactos/lib/sdk/crt/string/atoi.c
+++ b/reactos/lib/sdk/crt/string/atoi.c
@@ -8,5 +8,5 @@
 int
 _ttoi(const _TCHAR *str)
 {
-  return (int)_tcstol(str, 0, 10);
+  return (int)_ttoi64(str);
 }
diff --git a/reactos/lib/sdk/crt/string/atoi64.c b/reactos/lib/sdk/crt/string/atoi64.c
index d97d359ee6f..c820dbaba42 100644
--- a/reactos/lib/sdk/crt/string/atoi64.c
+++ b/reactos/lib/sdk/crt/string/atoi64.c
@@ -20,6 +20,9 @@ _atoi64(const char *nptr)
   __int64 acc = 0;
   int neg = 0;
 
+  if (nptr == NULL)
+    return 0;
+
   while(isspace((int)*s))
     s++;
   if (*s == '-')
diff --git a/reactos/lib/sdk/crt/string/atol.c b/reactos/lib/sdk/crt/string/atol.c
index cc3fe76cd0e..9aa3af57d87 100644
--- a/reactos/lib/sdk/crt/string/atol.c
+++ b/reactos/lib/sdk/crt/string/atol.c
@@ -8,7 +8,7 @@
 long
 _ttol(const _TCHAR *str)
 {
-  return _tcstol(str, 0, 10);
+  return (long)_ttoi64(str);
 }
 
 int _atoldbl(long double *value, const char *str)
diff --git a/reactos/lib/sdk/crt/string/wtoi64.c b/reactos/lib/sdk/crt/string/wtoi64.c
index 6e0f9dbdd64..b729f493a56 100644
--- a/reactos/lib/sdk/crt/string/wtoi64.c
+++ b/reactos/lib/sdk/crt/string/wtoi64.c
@@ -20,6 +20,9 @@ _wtoi64 (const wchar_t *nptr)
    __int64 value;
    int sign;
 
+   if (nptr == NULL)
+       return 0;
+
    while (iswctype((int)*nptr, _SPACE))
         ++nptr;
 
diff --git a/reactos/lib/sdk/crt/string/wtol.c b/reactos/lib/sdk/crt/string/wtol.c
index 9944956384b..a57015e9f7b 100644
--- a/reactos/lib/sdk/crt/string/wtol.c
+++ b/reactos/lib/sdk/crt/string/wtol.c
@@ -13,6 +13,9 @@ _wtol(const wchar_t *str)
     unsigned long RunningTotal = 0;
     char bMinus = 0;
 
+    if (str == NULL)
+        return 0;
+
     while (iswctype(*str, _SPACE) ) {
         str++;
     } /* while */