sv.h: Correct assertion in BmUSEFUL

BmUSEFUL uses the NV slot, not the IV slot.  So asserting that it is
not IOK is not all that useful.

1 files changed, 1 insertions, 1 deletions

diff --git a/sv.h b/sv.h
index 0ba73d18e7..98d712d5da 100644
--- a/sv.h
+++ b/sv.h
@@ -1361,7 +1361,7 @@ sv_force_normal does nothing.
 	(*({ SV *const _bmuseful = MUTABLE_SV(sv);			\
 	    assert(SvTYPE(_bmuseful) == SVt_PVMG);			\
 	    assert(SvVALID(_bmuseful));					\
-	    assert(!SvIOK(_bmuseful));					\
+	    assert(!SvNOK(_bmuseful));					\
 	    &(((XPVMG*) SvANY(_bmuseful))->xnv_u.xbm_useful);		\
 	 }))
 #else