diff --git a/coregrind/m_syswrap/syswrap-generic.c b/coregrind/m_syswrap/syswrap-generic.c
index d3b02d4b1..c86691dee 100644
--- a/coregrind/m_syswrap/syswrap-generic.c
+++ b/coregrind/m_syswrap/syswrap-generic.c
@@ -5275,7 +5275,10 @@ PRE(sys_sigaltstack)
    PRE_REG_READ2(int, "sigaltstack",
                  const vki_stack_t *, ss, vki_stack_t *, oss);
    if (ARG1 != 0) {
-      PRE_MEM_READ( "sigaltstack(ss)", ARG1, sizeof(vki_stack_t) );
+      const vki_stack_t *ss = (vki_stack_t *)ARG1;
+      PRE_MEM_READ( "sigaltstack(ss)", (Addr)&ss->ss_sp, sizeof(ss->ss_sp) );
+      PRE_MEM_READ( "sigaltstack(ss)", (Addr)&ss->ss_flags, sizeof(ss->ss_flags) );
+      PRE_MEM_READ( "sigaltstack(ss)", (Addr)&ss->ss_size, sizeof(ss->ss_size) );
    }
    if (ARG2 != 0) {
       PRE_MEM_WRITE( "sigaltstack(oss)", ARG2, sizeof(vki_stack_t) );