diff --git a/openpgp/read.go b/openpgp/read.go
index 94c4e6a..fa530dd 100644
--- a/openpgp/read.go
+++ b/openpgp/read.go
@@ -405,7 +405,10 @@ func CheckDetachedSignature(keyring KeyRing, signed, signature io.Reader) (signe
 		}
 	}
 
-	return nil, errors.ErrUnknownIssuer
+	if err == nil {
+		err = errors.ErrUnknownIssuer
+	}
+	return nil, err
 }
 
 // CheckArmoredDetachedSignature performs the same actions as
diff --git a/openpgp/read_test.go b/openpgp/read_test.go
index b08469d..796095c 100644
--- a/openpgp/read_test.go
+++ b/openpgp/read_test.go
@@ -279,6 +279,15 @@ func TestDetachedSignature(t *testing.T) {
 	testDetachedSignature(t, kring, readerFromHex(detachedSignatureHex), signedInput, "binary", testKey1KeyId)
 	testDetachedSignature(t, kring, readerFromHex(detachedSignatureTextHex), signedInput, "text", testKey1KeyId)
 	testDetachedSignature(t, kring, readerFromHex(detachedSignatureV3TextHex), signedInput, "v3", testKey1KeyId)
+
+	incorrectSignedInput := signedInput + "X"
+	_, err := CheckDetachedSignature(kring, bytes.NewBufferString(incorrectSignedInput), readerFromHex(detachedSignatureHex))
+	if err == nil {
+		t.Fatal("CheckDetachedSignature returned without error for bad signature")
+	}
+	if err == errors.ErrUnknownIssuer {
+		t.Fatal("CheckDetachedSignature returned ErrUnknownIssuer when the signer was known, but the signature invalid")
+	}
 }
 
 func TestDetachedSignatureDSA(t *testing.T) {