diff --git a/acme/http.go b/acme/http.go
index c51943e..2b4c1a1 100644
--- a/acme/http.go
+++ b/acme/http.go
@@ -10,6 +10,7 @@
 	"crypto"
 	"crypto/rand"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"math/big"
@@ -215,6 +216,9 @@
 func (c *Client) postNoRetry(ctx context.Context, key crypto.Signer, url string, body interface{}) (*http.Response, *http.Request, error) {
 	kid := noKeyID
 	if key == nil {
+		if c.Key == nil {
+			return nil, nil, errors.New("acme: Client.Key must be populated to make POST requests")
+		}
 		key = c.Key
 		kid = c.accountKID(ctx)
 	}
diff --git a/acme/http_test.go b/acme/http_test.go
index 79095cc..cf1df36 100644
--- a/acme/http_test.go
+++ b/acme/http_test.go
@@ -238,3 +238,18 @@
 		}
 	}
 }
+
+func TestAccountKidLoop(t *testing.T) {
+	// if Client.postNoRetry is called with a nil key argument
+	// then Client.Key must be set, otherwise we fall into an
+	// infinite loop (which also causes a deadlock).
+	client := &Client{dir: &Directory{OrderURL: ":)"}}
+	_, _, err := client.postNoRetry(context.Background(), nil, "", nil)
+	if err == nil {
+		t.Fatal("Client.postNoRetry didn't fail with a nil key")
+	}
+	expected := "acme: Client.Key must be populated to make POST requests"
+	if err.Error() != expected {
+		t.Fatalf("Unexpected error returned: wanted %q, got %q", expected, err.Error())
+	}
+}