diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index 8f4fea4..d4da4f5 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -531,7 +531,7 @@
 
 func (ctx *parseContext) handleAssignment(a *mkparser.Assignment) []starlarkNode {
 	// Handle only simple variables
-	if !a.Name.Const() {
+	if !a.Name.Const() || a.Target != nil {
 		return []starlarkNode{ctx.newBadNode(a, "Only simple variables are handled")}
 	}
 	name := a.Name.Strings[0]
diff --git a/mk2rbc/mk2rbc_test.go b/mk2rbc/mk2rbc_test.go
index de75129..6887ca4 100644
--- a/mk2rbc/mk2rbc_test.go
+++ b/mk2rbc/mk2rbc_test.go
@@ -1242,13 +1242,15 @@
 		desc:   "Ignore make rules",
 		mkname: "product.mk",
 		in: `
+foo: PRIVATE_VARIABLE = some_tool $< $@
 foo: foo.c
 	gcc -o $@ $*`,
 		expected: `load("//build/make/core:product_config.rbc", "rblf")
 
 def init(g, handle):
   cfg = rblf.cfg(handle)
-  rblf.mk2rbc_error("product.mk:2", "unsupported line rule:       foo: foo.c\n#gcc -o $@ $*")
+  rblf.mk2rbc_error("product.mk:2", "Only simple variables are handled")
+  rblf.mk2rbc_error("product.mk:3", "unsupported line rule:       foo: foo.c\n#gcc -o $@ $*")
 `,
 	},
 	{