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This is a maintenance category for articles with invalid ISSNs. It is sometimes added by the template {{Please check ISSN}}, which should be located next to the ISSN with the problem.

Articles can also be included in this category if an ISSN used in an {{ISSN}} or {{ISSN link}} template is not valid.

The most likely reason is a bad checksum, (see ISSN) which probably means there is a typing error in the ISSN. Also, ISSNs that are not eight digits in a 4-4 pattern (i.e.: ISSN 1234-5678) will return an error message. Use an upper-case "X", not a lower-case "x", if "X" is the final character in the ISSN. The two groups of four characters are joined with a hyphen, not a dash.

See also Category:CS1 errors: ISSN