diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index b43bf85..ced0768 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -1001,6 +1001,7 @@
 	size_t nfree;	/* # of free buffer slots; pvend-pvfree */
 	size_t total_v_size;  /* total # of slots in buffer */
 	size_t increment;	/* amount to increment the buffer */
+	size_t prev_v_size;
 
 	/* Optimize for normal case:  avoid _PyString_Resize if at all
 	 * possible via first reading into stack buffer "buf".
@@ -1115,8 +1116,11 @@
 		/* expand buffer and try again */
 		assert(*(pvend-1) == '\0');
 		increment = total_v_size >> 2;	/* mild exponential growth */
+		prev_v_size = total_v_size;
 		total_v_size += increment;
-		if (total_v_size > PY_SSIZE_T_MAX) {
+		/* check for overflow */
+		if (total_v_size <= prev_v_size ||
+		    total_v_size > PY_SSIZE_T_MAX) {
 			PyErr_SetString(PyExc_OverflowError,
 			    "line is longer than a Python string can hold");
 			Py_DECREF(v);
@@ -1125,7 +1129,7 @@
 		if (_PyString_Resize(&v, (int)total_v_size) < 0)
 			return NULL;
 		/* overwrite the trailing null byte */
-		pvfree = BUF(v) + (total_v_size - increment - 1);
+		pvfree = BUF(v) + (prev_v_size - 1);
 	}
 	if (BUF(v) + total_v_size != p)
 		_PyString_Resize(&v, p - BUF(v));