diff --git a/vendor/cmd/go/internal/modfetch/cache.go b/vendor/cmd/go/internal/modfetch/cache.go
index e07b784..be0adc0 100644
--- a/vendor/cmd/go/internal/modfetch/cache.go
+++ b/vendor/cmd/go/internal/modfetch/cache.go
@@ -146,8 +146,8 @@
 		rev = info.Version
 
 		text, err = r.r.GoMod(rev)
-		checkGoMod(r.path, rev, text)
 		if err == nil {
+			checkGoMod(r.path, rev, text)
 			if err := writeDiskGoMod(file, text); err != nil {
 				fmt.Fprintf(os.Stderr, "go: writing go.mod cache: %v\n", err)
 			}
diff --git a/vendor/cmd/go/internal/modfetch/fetch.go b/vendor/cmd/go/internal/modfetch/fetch.go
index df7adda..aff2cc8 100644
--- a/vendor/cmd/go/internal/modfetch/fetch.go
+++ b/vendor/cmd/go/internal/modfetch/fetch.go
@@ -153,6 +153,11 @@
 	return true
 }
 
+// emptyGoModHash is the hash of a 1-file tree containing a 0-length go.mod.
+// A bug caused us to write these into go.sum files for non-modules.
+// We detect and remove them.
+const emptyGoModHash = "h1:G7mAYYxgmS0lVkHyy2hEOLQCFB0DlQFTMLWggykrydY="
+
 // readGoSum parses data, which is the content of file,
 // and adds it to goSum.m. The goSum lock must be held.
 func readGoSum(file string, data []byte) {
@@ -174,6 +179,10 @@
 		if len(f) != 3 {
 			base.Fatalf("go: malformed go.sum:\n%s:%d: wrong number of fields %v", file, lineno, len(f))
 		}
+		if f[2] == emptyGoModHash {
+			// Old bug; drop it.
+			continue
+		}
 		mod := module.Version{Path: f[0], Version: f[1]}
 		goSum.m[mod] = append(goSum.m[mod], f[2])
 	}
diff --git a/vendor/cmd/go/mod_test.go b/vendor/cmd/go/mod_test.go
index 79b4370..565bbb2 100644
--- a/vendor/cmd/go/mod_test.go
+++ b/vendor/cmd/go/mod_test.go
@@ -1019,6 +1019,35 @@
 	tg.run("mod", "-sync") // ignores missing ziphash file for ordinary go.sum validation
 
 	tg.runFail("mod", "-verify") // explicit verify fails with missing ziphash
+
+	tg.run("mod", "-droprequire", "rsc.io/quote")
+	tg.run("list", "rsc.io/quote/buggy")
+	data, err = ioutil.ReadFile(tg.path("x/go.sum"))
+	if strings.Contains(string(data), "buggy") {
+		t.Fatalf("did not expect buggy in go.sum:\n%s", data)
+	}
+	if !strings.Contains(string(data), "rsc.io/quote v1.5.2/go.mod") {
+		t.Fatalf("did expect rsc.io/quote go.mod in go.sum:\n%s", data)
+	}
+
+	tg.run("mod", "-droprequire", "rsc.io/quote")
+	tg.runFail("list", "rsc.io/quote/buggy/foo")
+	data, err = ioutil.ReadFile(tg.path("x/go.sum"))
+	if strings.Contains(string(data), "buggy") {
+		t.Fatalf("did not expect buggy in go.sum:\n%s", data)
+	}
+	if !strings.Contains(string(data), "rsc.io/quote v1.5.2/go.mod") {
+		t.Fatalf("did expect rsc.io/quote go.mod in go.sum:\n%s", data)
+	}
+
+	tg.run("mod", "-droprequire", "rsc.io/quote")
+	tg.runFail("list", "rsc.io/quote/morebuggy")
+	if strings.Contains(string(data), "morebuggy") {
+		t.Fatalf("did not expect morebuggy in go.sum:\n%s", data)
+	}
+	if !strings.Contains(string(data), "rsc.io/quote v1.5.2/go.mod") {
+		t.Fatalf("did expect rsc.io/quote go.mod in go.sum:\n%s", data)
+	}
 }
 
 func TestModVendorNoDeps(t *testing.T) {