diff --git a/misc/cgo/testsanitizers/test.bash b/misc/cgo/testsanitizers/test.bash
index 88c54e6173..19407b8cb2 100755
--- a/misc/cgo/testsanitizers/test.bash
+++ b/misc/cgo/testsanitizers/test.bash
@@ -37,6 +37,11 @@ fi
 
 status=0
 
+if ! go build -msan std; then
+  echo "FAIL: build -msan std"
+  status=1
+fi
+
 if ! go run -msan msan.go; then
   echo "FAIL: msan"
   status=1
diff --git a/src/cmd/compile/internal/gc/racewalk.go b/src/cmd/compile/internal/gc/racewalk.go
index 7770f741df..acebb1ac9c 100644
--- a/src/cmd/compile/internal/gc/racewalk.go
+++ b/src/cmd/compile/internal/gc/racewalk.go
@@ -502,13 +502,25 @@ func callinstr(np **Node, init **NodeList, wr int, skip int) bool {
 			if wr != 0 {
 				name = "msanwrite"
 			}
-			f = mkcall(name, nil, init, uintptraddr(n), Nodintconst(t.Width))
+			// dowidth may not have been called for PEXTERN.
+			dowidth(t)
+			w := t.Width
+			if w == BADWIDTH {
+				Fatalf("instrument: %v badwidth", t)
+			}
+			f = mkcall(name, nil, init, uintptraddr(n), Nodintconst(w))
 		} else if flag_race != 0 && (t.Etype == TSTRUCT || Isfixedarray(t)) {
 			name := "racereadrange"
 			if wr != 0 {
 				name = "racewriterange"
 			}
-			f = mkcall(name, nil, init, uintptraddr(n), Nodintconst(t.Width))
+			// dowidth may not have been called for PEXTERN.
+			dowidth(t)
+			w := t.Width
+			if w == BADWIDTH {
+				Fatalf("instrument: %v badwidth", t)
+			}
+			f = mkcall(name, nil, init, uintptraddr(n), Nodintconst(w))
 		} else if flag_race != 0 {
 			name := "raceread"
 			if wr != 0 {