diff --git a/Modules/_csv.c b/Modules/_csv.c
index 701904a..7a3bfc4 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -647,7 +647,7 @@
                         Py_DECREF(lineobj);
                         return NULL;
                 }
-		if (strlen(line) < PyString_GET_SIZE(lineobj)) {
+		if (strlen(line) < (size_t)PyString_GET_SIZE(lineobj)) {
 			self->had_parse_error = 1;
 			Py_DECREF(lineobj);
 			return PyErr_Format(error_obj,