diff --git a/Python/mysnprintf.c b/Python/mysnprintf.c
index e6c3fce..a373f4e 100644
--- a/Python/mysnprintf.c
+++ b/Python/mysnprintf.c
@@ -37,9 +37,10 @@
 	return len;
     }
     len++;
-    if (len > size + 512)
+    assert(len >= 0);
+    if ((size_t)len > size + 512)
 	Py_FatalError("Buffer overflow in PyOS_snprintf/PyOS_vsnprintf");
-    if (len > size) {
+    if ((size_t)len > size) {
 	PyMem_Free(buffer);
 	return len - 1;
     }