diff --git a/builtin/tag.c b/builtin/tag.c
index 3918eacbb5..b28ead06ea 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -176,7 +176,7 @@ static int verify_tag(const char *name, const char *ref UNUSED,
 
 static int do_sign(struct strbuf *buffer)
 {
-	return sign_buffer(buffer, buffer, get_signing_key());
+	return sign_buffer(buffer, buffer, get_signing_key()) ? -1 : 0;
 }
 
 static const char tag_template[] =
diff --git a/gpg-interface.h b/gpg-interface.h
index 143cdc1c02..7cd98161f7 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -66,7 +66,7 @@ size_t parse_signed_buffer(const char *buf, size_t size);
  * Create a detached signature for the contents of "buffer" and append
  * it after "signature"; "buffer" and "signature" can be the same
  * strbuf instance, which would cause the detached signature appended
- * at the end.
+ * at the end.  Returns 0 on success, non-zero on failure.
  */
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
 		const char *signing_key);