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BigQuery - UPDATE/MERGE error

i have two tables. Table1 = dalio which is an event list with select customers. Table2 = master_list which is a master customer list from all past events.

dalio has an "id" column that needs to be filled in with customer numbers, which can be pulled from master_list column called "customer_no". All rows in the "id" column are currently blank. I only want the customer numbers where the "fullname" column in dalio & "name" column in master_list are an exact match.

This is my query:
lce-tess.Tess_Attributes.dalio a
a.id = CAST(b.customer_no AS STRING)
lce-tess.Tess_Attributes.master_list b
a.fullname = b.name

Once run, I receive the following message: "UPDATE/MERGE must match at most one source row for each target row" --not sure how to troubleshoot.