In Perl_sv_setsv_cow(), dstr can be NULL.

Author: nwc10

embed.fnc

@@ -1866,7 +1866,7 @@ Apn	|int	|my_socketpair	|int family|int type|int protocol|int fd[2]
 Ap	|int	|my_dirfd	|NULLOK DIR* dir
 #ifdef PERL_OLD_COPY_ON_WRITE
 : Used in pp_hot.c and regexec.c
-pMXE	|SV*	|sv_setsv_cow	|NN SV* dstr|NN SV* sstr
+pMXE	|SV*	|sv_setsv_cow	|NULLOK SV* dstr|NN SV* sstr
 #endif
 
 Aop	|const char *|PerlIO_context_layers|NULLOK const char *mode

proto.h

@@ -5968,10 +5968,9 @@ PERL_CALLCONV int	Perl_my_socketpair(int family, int type, int protocol, int fd[
 PERL_CALLCONV int	Perl_my_dirfd(pTHX_ DIR* dir);
 #ifdef PERL_OLD_COPY_ON_WRITE
 PERL_CALLCONV SV*	Perl_sv_setsv_cow(pTHX_ SV* dstr, SV* sstr)
-			__attribute__nonnull__(pTHX_1)
 			__attribute__nonnull__(pTHX_2);
 #define PERL_ARGS_ASSERT_SV_SETSV_COW	\
-	assert(dstr); assert(sstr)
+	assert(sstr)
 
 #endif