diff --git a/libc/arch-arm/syscalls/__ptrace.S b/libc/arch-arm/syscalls/__ptrace.S
index 72acd70..984d499 100644
--- a/libc/arch-arm/syscalls/__ptrace.S
+++ b/libc/arch-arm/syscalls/__ptrace.S
@@ -1,4 +1,5 @@
 /* autogenerated by gensyscalls.py */
+#include <linux/err.h>
 #include <machine/asm.h>
 #include <sys/linux-syscalls.h>
 
@@ -7,7 +8,8 @@
     ldr     r7, =__NR_ptrace
     swi     #0
     mov     r7, ip
-    movs    r0, r0
-    bxpl    lr
-    b       __set_syscall_errno
+    cmn     r0, #(MAX_ERRNO + 1)
+    bxls    lr
+    neg     r0, r0
+    b       __set_errno
 END(__ptrace)