MySQL Error Number: 1452 (unusual case) - mysql

Question

I tried adding a Foreign Key using this query:
ALTER TABLE `StripeBilling`
ADD CONSTRAINT `StripeBillingPatientRef`
FOREIGN KEY (PatientRef)
REFERENCES `Patient`(`PatientId`)
ON DELETE SET NULL;
My tables mysql_dump is:
CREATE TABLE IF NOT EXISTS `StripeBilling` (
...
`PatientRef` int(10) unsigned DEFAULT NULL,
...
) ENGINE=InnoDB AUTO_INCREMENT=10000000 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
ALTER TABLE `StripeBilling`
ADD PRIMARY KEY (`StripeBillingId`), ADD KEY `StripeBillingAgentRef` (`AgentRef`), ADD KEY `PatientRef` (`PatientRef`);
ALTER TABLE `StripeBilling`
MODIFY `StripeBillingId` int(11) NOT NULL AUTO_INCREMENT,AUTO_INCREMENT=10000000;
ALTER TABLE `StripeBilling`
ADD CONSTRAINT `StripeBillingAgentRef` FOREIGN KEY (`AgentRef`) REFERENCES `Agent` (`AgentId`) ON DELETE SET NULL;
And patient's table is:
CREATE TABLE IF NOT EXISTS `Patient` (
`PatientId` int(10) unsigned NOT NULL,
...
) ENGINE=InnoDB AUTO_INCREMENT=10000000 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
ALTER TABLE `Patient`
ADD PRIMARY KEY (`PatientId`), ADD UNIQUE KEY `PatientSerial` (`PatientSerial`);
ALTER TABLE `Patient`
MODIFY `PatientId` int(10) unsigned NOT NULL AUTO_INCREMENT,AUTO_INCREMENT=10000000;
But MySQL returns an error:
Cannot add or update a child row: a foreign key constraint fails
(newportal_enc.#sql-3863_9f, CONSTRAINT StripeBillingPatientRef
FOREIGN KEY (PatientRef) REFERENCES Patient (PatientId) ON
DELETE SET NULL)
I'm confused because table newportal_enc.#sql-3863_9f does not exist in my database.
This is not due to the presence of the PatientRef index, becouse after removing of this index my issue still exists.
Why am I getting this error and how can I solve it?