diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 3ba52353283..0990c4563be 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -552,7 +552,11 @@ func disallowVendorVisibility(srcDir string, p *Package, stk *importStack) *Pack
 	if i > 0 {
 		i-- // rewind over slash in ".../vendor"
 	}
-	parent := p.Dir[:i+len(p.Dir)-len(p.ImportPath)]
+	truncateTo := i + len(p.Dir) - len(p.ImportPath)
+	if truncateTo < 0 || len(p.Dir) < truncateTo {
+		return p
+	}
+	parent := p.Dir[:truncateTo]
 	if hasPathPrefix(filepath.ToSlash(srcDir), filepath.ToSlash(parent)) {
 		return p
 	}
diff --git a/src/cmd/go/testdata/src/vend/x/invalid/invalid.go b/src/cmd/go/testdata/src/vend/x/invalid/invalid.go
new file mode 100644
index 00000000000..e250d5bb31b
--- /dev/null
+++ b/src/cmd/go/testdata/src/vend/x/invalid/invalid.go
@@ -0,0 +1,3 @@
+package invalid
+
+import "vend/x/invalid/vendor/foo"
diff --git a/src/cmd/go/vendor_test.go b/src/cmd/go/vendor_test.go
index 5fe5aaa91bb..3b99a29d98c 100644
--- a/src/cmd/go/vendor_test.go
+++ b/src/cmd/go/vendor_test.go
@@ -29,6 +29,7 @@ func TestVendorImports(t *testing.T) {
 		vend/vendor/q []
 		vend/vendor/strings []
 		vend/x [vend/x/vendor/p vend/vendor/q vend/x/vendor/r]
+		vend/x/invalid [vend/x/invalid/vendor/foo]
 		vend/x/vendor/p []
 		vend/x/vendor/p/p [notfound]
 		vend/x/vendor/r []
@@ -64,6 +65,16 @@ func TestVendorTest(t *testing.T) {
 	tg.grepStdout("TestMsgExternal", "missing use in external test")
 }
 
+func TestVendorInvalid(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.setenv("GO15VENDOREXPERIMENT", "1")
+
+	tg.runFail("build", "vend/x/invalid")
+	tg.grepStderr("must be imported as foo", "missing vendor import error")
+}
+
 func TestVendorImportError(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()