src/runtime/mbitmap.go  | 2 +-
 src/runtime/runtime1.go | 5 +++--

diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index b9b52a7bc416f8974a465534a9b9fa2579d0033c..c439158f078080f50d43685838cd24e6df01677d 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -201,7 +201,7 @@
 		// The following ensures that we are rigorous about what data
 		// structures hold valid pointers.
 		// TODO(rsc): Check if this still happens.
-		if true {
+		if debug.invalidptr != 0 {
 			// Still happens sometimes. We don't know why.
 			printlock()
 			print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index db269957033dc46d491f3fe54ca3cf192477b3dd..a50e5b618cfae4c7c19963e9eba3ed3af47e76d7 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -299,8 +299,6 @@ 	name  string
 	value *int32
 }
 
-// TODO(rsc): Make GC respect debug.invalidptr.
-
 // Holds variables parsed from GODEBUG env var,
 // except for "memprofilerate" since there is an
 // existing int var for that value, which may
@@ -340,6 +338,9 @@ 	{"wbshadow", &debug.wbshadow},
 }
 
 func parsedebugvars() {
+	// defaults
+	debug.invalidptr = 1
+
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""
 		i := index(p, ",")