diff --git a/signature/fixtures/unknown-key.signature b/signature/fixtures/unknown-key.signature
new file mode 100644
index 00000000..2277b130
Binary files /dev/null and b/signature/fixtures/unknown-key.signature differ
diff --git a/signature/mechanism_test.go b/signature/mechanism_test.go
index 8617b6d2..e6cbc0a7 100644
--- a/signature/mechanism_test.go
+++ b/signature/mechanism_test.go
@@ -138,5 +138,12 @@ func TestGPGSigningMechanismVerify(t *testing.T) {
 	require.NoError(t, err)
 	content, signingFingerprint, err = mech.Verify(signature)
 	assertSigningError(t, content, signingFingerprint, err)
+
+	// Valid signature with an unknown key
+	signature, err = ioutil.ReadFile("./fixtures/unknown-key.signature")
+	require.NoError(t, err)
+	content, signingFingerprint, err = mech.Verify(signature)
+	assertSigningError(t, content, signingFingerprint, err)
+
 	// The various GPG/GPGME failures cases are not obviously easy to reach.
 }