diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index 139a4a8..237d8c2 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -119,6 +119,7 @@
 static PyObject *
 IO_cgetval(PyObject *self) {
         if (!IO__opencheck(IOOOBJECT(self))) return NULL;
+        assert(IOOOBJECT(self)->pos >= 0);
         return PyString_FromStringAndSize(((IOobject*)self)->buf,
                                           ((IOobject*)self)->pos);
 }
@@ -137,6 +138,7 @@
         }
         else
                   s=self->string_size;
+        assert(self->pos >= 0);
         return PyString_FromStringAndSize(self->buf, s);
 }
 
@@ -157,6 +159,8 @@
         Py_ssize_t l;
 
         if (!IO__opencheck(IOOOBJECT(self))) return -1;
+        assert(IOOOBJECT(self)->pos >= 0);
+        assert(IOOOBJECT(self)->string_size >= 0);
         l = ((IOobject*)self)->string_size - ((IOobject*)self)->pos;  
         if (n < 0 || n > l) {
                 n = l;
@@ -192,12 +196,17 @@
         for (n = ((IOobject*)self)->buf + ((IOobject*)self)->pos,
                s = ((IOobject*)self)->buf + ((IOobject*)self)->string_size; 
              n < s && *n != '\n'; n++);
+
         if (n < s) n++;
 
         *output=((IOobject*)self)->buf + ((IOobject*)self)->pos;
         l = n - ((IOobject*)self)->buf - ((IOobject*)self)->pos;
-	assert(((IOobject*)self)->pos + l < INT_MAX);
-        ((IOobject*)self)->pos += (int)l;
+
+        assert(IOOOBJECT(self)->pos <= PY_SSIZE_T_MAX - l);
+        assert(IOOOBJECT(self)->pos >= 0);
+        assert(IOOOBJECT(self)->string_size >= 0);
+
+        ((IOobject*)self)->pos += l;
         return (int)l;
 }
 
@@ -215,6 +224,7 @@
                 n -= m;
                 self->pos -= m;
         }
+        assert(IOOOBJECT(self)->pos >= 0);
         return PyString_FromStringAndSize(output, n);
 }
 
@@ -277,6 +287,7 @@
 
         if (!IO__opencheck(self)) return NULL;
 
+        assert(self->pos >= 0);
         return PyInt_FromSsize_t(self->pos);
 }