diff --git a/Makefile-rpm-ostree.am b/Makefile-rpm-ostree.am
index 60e27b3f..7a90a44a 100644
--- a/Makefile-rpm-ostree.am
+++ b/Makefile-rpm-ostree.am
@@ -44,7 +44,7 @@ rpm_ostree_SOURCES = src/app/main.cxx \
 	src/app/rpmostree-builtin-db.cxx \
 	src/app/rpmostree-builtin-start-daemon.cxx \
 	src/app/rpmostree-builtin-finalize-deployment.cxx \
-	src/app/rpmostree-db-builtin-diff.c \
+	src/app/rpmostree-db-builtin-diff.cxx \
 	src/app/rpmostree-db-builtin-list.c \
 	src/app/rpmostree-db-builtin-version.c \
 	src/app/rpmostree-dbus-helpers.cxx \
diff --git a/src/app/rpmostree-db-builtin-diff.c b/src/app/rpmostree-db-builtin-diff.cxx
similarity index 98%
rename from src/app/rpmostree-db-builtin-diff.c
rename to src/app/rpmostree-db-builtin-diff.cxx
index 54db223a..66b86452 100644
--- a/src/app/rpmostree-db-builtin-diff.c
+++ b/src/app/rpmostree-db-builtin-diff.cxx
@@ -30,7 +30,7 @@
 #include "rpmostree-rpm-util.h"
 #include "rpmostree-package-variants.h"
 
-static char *opt_format = "block";
+static char *opt_format;
 static gboolean opt_changelogs;
 static char *opt_sysroot;
 static gboolean opt_base;
@@ -117,7 +117,7 @@ get_checksum_from_deployment (OstreeRepo       *repo,
   if (!checksum)
     checksum = g_strdup (ostree_deployment_get_csum (deployment));
 
-  *out_checksum = g_steal_pointer (&checksum);
+  *out_checksum = util::move_nullify (checksum);
   return TRUE;
 }
 
@@ -162,6 +162,9 @@ rpmostree_db_builtin_diff (int argc, char **argv,
       return FALSE;
     }
 
+  if (!opt_format)
+    opt_format = g_strdup ("block");
+
   if (g_str_equal (opt_format, "json") && opt_changelogs)
     {
       rpmostree_usage_error (context, "json format and --changelogs not supported", error);