diff --git a/PC/winreg.c b/PC/winreg.c
index 37bc2c72dcd..72a7c380bee 100644
--- a/PC/winreg.c
+++ b/PC/winreg.c
@@ -772,7 +772,7 @@ Reg2Py(BYTE *retDataBuf, DWORD retDataSize, DWORD typ)
                         return NULL;
                     }
                     PyList_SET_ITEM(obData, index, uni);
-                    len -= slen + 1;
+                    len -= Py_SAFE_DOWNCAST(slen + 1, size_t, int);
                 }
                 PyMem_Free(str);