diff --git a/src/lib-mail/message-part-data.c b/src/lib-mail/message-part-data.c
index 779d8107b5..ffae458f55 100644
--- a/src/lib-mail/message-part-data.c
+++ b/src/lib-mail/message-part-data.c
@@ -25,6 +25,7 @@ bool message_part_data_is_plain_7bit(const struct message_part *part)
 {
 	const struct message_part_data *data = part->data;
 
+	i_assert(data != NULL);
 	i_assert(part->parent == NULL);
 
 	/* if content-type is text/xxx we don't have to check any
@@ -73,6 +74,8 @@ bool message_part_data_get_filename(const struct message_part *part,
 	const struct message_part_param *params;
 	unsigned int params_count, i;
 
+	i_assert(data != NULL);
+
 	params = data->content_disposition_params;
 	params_count = data->content_disposition_params_count;
 
@@ -539,6 +542,8 @@ bool message_part_has_content_types(struct message_part *part,
 	const char *const *ptr;
 	const char *content_type;
 
+	i_assert(data != NULL);
+
 	if (data->content_type == NULL)
 		return FALSE;
 	else if (data->content_subtype == NULL)
@@ -560,6 +565,8 @@ bool message_part_has_parameter(struct message_part *part, const char *parameter
 {
 	struct message_part_data *data = part->data;
 
+	i_assert(data != NULL);
+
 	for (unsigned int i = 0; i < data->content_disposition_params_count; i++) {
 		const struct message_part_param *param =
 			&data->content_disposition_params[i];