diff --git a/builtin/log.c b/builtin/log.c
index 734aab3a731..cb14db48987 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -705,7 +705,7 @@ static int git_format_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 	if (!strcmp(var, "diff.color") || !strcmp(var, "color.diff") ||
-	    !strcmp(var, "color.ui")) {
+	    !strcmp(var, "color.ui") || !strcmp(var, "diff.submodule")) {
 		return 0;
 	}
 	if (!strcmp(var, "format.numbered")) {
diff --git a/t/t4255-am-submodule.sh b/t/t4255-am-submodule.sh
index 450d26136a0..0ba8194403f 100755
--- a/t/t4255-am-submodule.sh
+++ b/t/t4255-am-submodule.sh
@@ -80,12 +80,12 @@ test_expect_success 'diff.submodule unset with extra file' '
 	run_test $THIRD second-submodule
 '
 
-test_expect_failure 'diff.submodule=log' '
+test_expect_success 'diff.submodule=log' '
 	test_config diff.submodule log &&
 	run_test $SECOND first-submodule
 '
 
-test_expect_failure 'diff.submodule=log with extra file' '
+test_expect_success 'diff.submodule=log with extra file' '
 	test_config diff.submodule log &&
 	run_test $THIRD second-submodule
 '