Mantis importer breaks with String custom field types

Error importing data from Mantis.java.lang.RuntimeException: class
com.atlassian.jira.issue.customfields.impl.RenderableTextCFType passed
an invalid value of type: class java.util.ArrayList at
com.atlassian.jira.issue.customfields.impl.AbstractCustomFieldType.asser
tObjectImplementsType(AbstractCustomFieldType.java:92) at
com.atlassian.jira.issue.customfields.impl.TextCFType.getStringFromSingu
larObject(TextCFType.java:24) at
com.atlassian.jira.issue.customfields.impl.StringCFType.getDbValueFromOb
ject(StringCFType.java:20) at
com.atlassian.jira.issue.customfields.impl.AbstractSingleFieldType.creat
eValue(AbstractSingleFieldType.java:119) at
com.atlassian.jira.issue.fields.CustomFieldImpl.createValue(CustomFieldI
mpl.java:436) at
com.atlassian.jira.util.MantisImportBean.importCustomFieldValues(MantisI
mportBean.java:1260) at
com.atlassian.jira.util.MantisImportBean.importIssues(MantisImportBean.j
ava:536) at
com.atlassian.jira.util.MantisImportBean.importProjects(MantisImportBean
.java:1017) at
com.atlassian.jira.util.MantisImportBean.doImport(MantisImportBean.java:
259) at
com.atlassian.jira.web.action.util.MantisImport.doRun(MantisImport.java:
149) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at