diff --git a/internal/proxy/passthrough_test.go b/internal/proxy/passthrough_test.go
index 46cb72d..80dfdb3 100644
--- a/internal/proxy/passthrough_test.go
+++ b/internal/proxy/passthrough_test.go
@@ -151,6 +151,41 @@ func wantRequestFields(t *testing.T, line logLine, host string, sent, received i
 	}
 }
 
+func TestAddsNoContentTypeTheAppDidNotSend(t *testing.T) {
+	t.Parallel()
+
+	const typed = "text/plain"
+
+	app := startApp(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/typed" {
+			w.Header().Set("Content-Type", typed)
+		} else {
+			// The key with no value keeps Go's server from adding one.
+			w.Header()["Content-Type"] = nil
+		}
+
+		_, _ = io.WriteString(w, "<!doctype html><title>page</title>")
+	})
+	addr, _ := startProxy(t, app.URL, nil)
+
+	// Go's server guesses a Content-Type for an answer without one only
+	// when the first part of the body goes out with the headers, which
+	// happens to some answers and not others: the test asks many times.
+	for range 50 {
+		got := get(t, addr, "/")
+		if got.header.Values("Content-Type") != nil {
+			t.Fatalf("client got Content-Type %q, want none",
+				got.header.Get("Content-Type"))
+		}
+	}
+
+	got := get(t, addr, "/typed")
+	if got.header.Get("Content-Type") != typed {
+		t.Errorf("client got Content-Type %q, want %q",
+			got.header.Get("Content-Type"), typed)
+	}
+}
+
 func TestStreamsTheRequestBody(t *testing.T) {
 	t.Parallel()
 
diff --git a/internal/proxy/request.go b/internal/proxy/request.go
index e67599c..922afa5 100644
--- a/internal/proxy/request.go
+++ b/internal/proxy/request.go
@@ -421,6 +421,13 @@ func (rq *request) modifyResponse(res *http.Response) error {
 		return errResponseTooLarge
 	}
 
+	// Go's server adds a Content-Type it guesses from the body to a
+	// response whose headers have no Content-Type key. The key with no
+	// value sends none, as the app sent none.
+	if res.Header.Get("Content-Type") == "" {
+		rq.out.Header()["Content-Type"] = nil
+	}
+
 	res.Body = &responseBody{body: limitBody(res.Body, maxBytes), rq: rq}
 	rq.startClientResponseTimeout()