I'm getting the error below for what I believe to be a valid mapping. Am I doing something wrong? Any ideas?

org.hibernate.MappingException: Foreign key (FKFA49A91FAB488FB6:billing_document_lines [line_number])) must have same number of columns as the referenced primary key (document_lines [line_number,document_id])

Here's the code:

BillingDocument.java

package document;

import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "billing_documents")
public class BillingDocument extends Document {

 private static final long serialVersionUID = 1L;

}

package document;

import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "billing_document_lines")
public class BillingDocumentLine extends DocumentLine {

 private static final long serialVersionUID = 1L;

 public BillingDocumentLine(BillingDocument document, short lineNumber) {
 super(document, lineNumber);
 }

 BillingDocumentLine() {
 }

}

package document;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Id;
import javax.persistence.Entity;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;

@Entity
@Table(name = "documents")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Document implements Serializable {

 private static final long serialVersionUID = 1L;

 protected String documentID;

 protected Document() {
 }

 protected Document(String documentID) {
 setDocumentID(documentID);
 }

 @Override
 public int hashCode() {
 final int PRIME = 31;
 int result = 1;
 result = PRIME * result
 + ((documentID == null) ? 0 : documentID.hashCode());
 return result;
 }

 @Override
 public boolean equals(Object object) {
 if (object instanceof Document) {
 final Document document = (Document) object;
 if (documentID != null) {
 return documentID.equals(document.documentID);
 }
 }
 return false;
 }

 @Override
 public String toString() {
 return documentID;
 }

 @Id
 @Column(name = "document_id")
 public String getDocumentID() {
 return documentID;
 }

 protected void setDocumentID(String documentID) {
 this.documentID = documentID;
 }
}

package document;

import java.io.Serializable;

import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;

@Entity
@Table(name = "document_lines")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class DocumentLine implements Serializable {

 private static final long serialVersionUID = 1L;

 protected DocumentLinePK primaryKey;

 protected DocumentLine(Document document, short lineNumber) {
 setPrimaryKey(new DocumentLinePK(document, lineNumber));
 }

 protected DocumentLine() {
 }

 @Override
 public int hashCode() {
 final int PRIME = 31;
 int result = 1;
 result = PRIME * result
 + ((primaryKey == null) ? 0 : primaryKey.hashCode());
 return result;
 }

 @Override
 public boolean equals(Object object) {
 if (object instanceof DocumentLine) {
 final DocumentLine documentLine = (DocumentLine) object;
 if (primaryKey != null) {
 return primaryKey.equals(documentLine.primaryKey);
 }
 }
 return false;
 }

 @Override
 public String toString() {
 return primaryKey.toString();
 }

 @EmbeddedId
 public DocumentLinePK getPrimaryKey() {
 return primaryKey;
 }

 protected void setPrimaryKey(DocumentLinePK primaryKey) {
 this.primaryKey = primaryKey;
 }
}

package document;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;
import javax.persistence.FetchType;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;

@Embeddable
public class DocumentLinePK implements Serializable {

 private static final long serialVersionUID = 1L;

 private Document document;

 private short lineNumber;

 public DocumentLinePK(Document document, short lineNumber) {
 setDocument(document);
 setLineNumber(lineNumber);
 }

 DocumentLinePK() {
 }

 @Override
 public int hashCode() {
 final int PRIME = 31;
 int result = 1;
 result = PRIME * result
 + ((document == null) ? 0 : document.hashCode());
 result = PRIME * result + lineNumber;
 return result;
 }

 @Override
 public boolean equals(Object object) {
 if (object == null) {
 return false;
 }
 if (getClass() == object.getClass()) {
 final DocumentLinePK primaryKey = (DocumentLinePK) object;
 if (document != null && lineNumber != 0) {
 return document.equals(primaryKey.document)
 && lineNumber == primaryKey.lineNumber;
 }
 }
 return false;
 }

 @Override
 public String toString() {
 return document.toString() + ":" + lineNumber;
 }

 @ManyToOne(optional = false, fetch = FetchType.LAZY)
 @JoinColumn(name = "document_id")
 public Document getDocument() {
 return document;
 }

 @Column(name = "line_number")
 public short getLineNumber() {
 return lineNumber;
 }

 protected void setDocument(Document document) {
 this.document = document;
 }

 protected void setLineNumber(short lineNumber) {
 this.lineNumber = lineNumber;
 }
}