diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index 3a6854c29..78ab771b9 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -635,6 +635,13 @@ func (ctx *parseContext) handleAssignment(a *mkparser.Assignment) []starlarkNode
 	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))
diff --git a/mk2rbc/mk2rbc_test.go b/mk2rbc/mk2rbc_test.go
index a9b61978a..0c4d21375 100644
--- a/mk2rbc/mk2rbc_test.go
+++ b/mk2rbc/mk2rbc_test.go
@@ -923,8 +923,6 @@ def init(g, handle):
   cfg["PRODUCT_LIST2"] += ["a"]
   cfg["PRODUCT_LIST1"] += ["b"]
   cfg["PRODUCT_LIST2"] += ["b"]
-  if cfg.get("PRODUCT_LIST3") == None:
-    cfg["PRODUCT_LIST3"] = ["a"]
   cfg["PRODUCT_LIST1"] = ["c"]
   g.setdefault("PLATFORM_LIST", [])
   g["PLATFORM_LIST"] += ["x"]
@@ -966,9 +964,10 @@ PRODUCT_LIST1 = a $(PRODUCT_LIST1)
 PRODUCT_LIST2 ?= a $(PRODUCT_LIST2)
 PRODUCT_LIST3 += a
 
-# Now doing them again should not have a setdefault because they've already been set
+# Now doing them again should not have a setdefault because they've already been set, except 2
+# which did not emit an assignment before
 PRODUCT_LIST1 = a $(PRODUCT_LIST1)
-PRODUCT_LIST2 ?= a $(PRODUCT_LIST2)
+PRODUCT_LIST2 = a $(PRODUCT_LIST2)
 PRODUCT_LIST3 += a
 `,
 		expected: `# All of these should have a setdefault because they're self-referential and not defined before
@@ -979,18 +978,15 @@ def init(g, handle):
   rblf.setdefault(handle, "PRODUCT_LIST1")
   cfg["PRODUCT_LIST1"] = (["a"] +
       cfg.get("PRODUCT_LIST1", []))
-  if cfg.get("PRODUCT_LIST2") == None:
-    rblf.setdefault(handle, "PRODUCT_LIST2")
-    cfg["PRODUCT_LIST2"] = (["a"] +
-        cfg.get("PRODUCT_LIST2", []))
   rblf.setdefault(handle, "PRODUCT_LIST3")
   cfg["PRODUCT_LIST3"] += ["a"]
-  # Now doing them again should not have a setdefault because they've already been set
+  # Now doing them again should not have a setdefault because they've already been set, except 2
+  # which did not emit an assignment before
   cfg["PRODUCT_LIST1"] = (["a"] +
       cfg["PRODUCT_LIST1"])
-  if cfg.get("PRODUCT_LIST2") == None:
-    cfg["PRODUCT_LIST2"] = (["a"] +
-        cfg["PRODUCT_LIST2"])
+  rblf.setdefault(handle, "PRODUCT_LIST2")
+  cfg["PRODUCT_LIST2"] = (["a"] +
+      cfg.get("PRODUCT_LIST2", []))
   cfg["PRODUCT_LIST3"] += ["a"]
 `,
 	},
diff --git a/mk2rbc/variable.go b/mk2rbc/variable.go
index 0a26ed8b2..95e1f8ec7 100644
--- a/mk2rbc/variable.go
+++ b/mk2rbc/variable.go
@@ -109,14 +109,11 @@ func (pcv productConfigVariable) emitSet(gctx *generationContext, asgn *assignme
 		}
 		emitAppend()
 	case asgnMaybeSet:
-		gctx.writef("if cfg.get(%q) == None:", pcv.nam)
-		gctx.indentLevel++
-		gctx.newLine()
-		if needsSetDefault {
-			emitSetDefault()
-		}
-		emitAssignment()
-		gctx.indentLevel--
+		// In mk2rbc.go we never emit a maybeSet assignment for product config variables, because
+		// they are set to empty strings before running product config.
+		panic("Should never get here")
+	default:
+		panic("Unknown assignment flavor")
 	}
 
 	gctx.setHasBeenAssigned(&pcv)