Liverpool centre-back Mamadou Sakho was suspended last season for taking a substance that was not on the World Anti-Doping Agency's (Wada) banned list, according to a Uefa report.
European football's governing body blamed "gaps in communication" between Wada and two of its laboratories.
A Uefa report released on Thursday said it was "clearly not possible" for anyone to tell whether higenamine is a prohibited substance by reading Wada's banned list.
"The fact that the Cologne laboratory tested for higenamine but had to check with Wada before making a determination indicates a problem, as does the fact that the Lausanne laboratory does not test for it," it added.
"The onus is clearly on Wada to communicate to its laboratories what is and what is not on the prohibited list.
"There are clearly gaps in communication with regard to higenamine, something which also tends to support the suggestion that Wada's own internal procedure and analysis in respect of this substance is incomplete."
Sakho, 27, admitted taking the substance without Liverpool's knowledge but contended it was not on the banned list, which was supported by the report into the case by Uefa's control, ethics and disciplinary body.
The France international has not played for Liverpool since testing positive and moved to Palace on loan in February.