diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index d905ba2..d2d1698 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -581,7 +581,7 @@
         return NULL;
     }
 
-    if (strlen(str) != size) {
+    if (strlen(str) != (size_t)size) {
         PyErr_SetString(PyExc_TypeError,
                         "source code string cannot contain null bytes");
         return NULL;