diff --git a/PC/winreg.c b/PC/winreg.c
index d62a7be28d3..fb488d8eb02 100644
--- a/PC/winreg.c
+++ b/PC/winreg.c
@@ -680,7 +680,7 @@ _Py_COMP_DIAG_POP
                     assert(size > 0);
                     len = PyUnicode_AsWideChar(t, P, size);
                     assert(len >= 0);
-                    assert(len < size);
+                    assert((unsigned)len < size);
                     size -= (DWORD)len + 1;
                     P += len + 1;
                 }