undefined method `sourcedb' for nil

PMC:7536900 / 233-239 JSONTXT

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{"target":"https://pubannotation.org/docs/sourcedb/PMC/sourceid/7536900","sourcedb":"PMC","sourceid":"7536900","source_url":"https://www.ncbi.nlm.nih.gov/pmc/7536900","text":"ymptom","tracks":[]}