diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index 3a6854c..78ab771 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -635,6 +635,13 @@
 	case "+=":
 		asgn.flavor = asgnAppend
 	case "?=":
+		if _, ok := lhs.(*productConfigVariable); ok {
+			// Make sets all product configuration variables to empty strings before running product
+			// config makefiles. ?= will have no effect on a variable that has been assigned before,
+			// even if assigned to an empty string. So just skip emitting any code for this
+			// assignment.
+			return nil
+		}
 		asgn.flavor = asgnMaybeSet
 	default:
 		panic(fmt.Errorf("unexpected assignment type %s", a.Type))