[SQL Server] Incorrect syntax near the keyword 'THEN'

[likeamelody]

Ciao, eseguendo la seguente query

query = "SELECT col1, col2, col3, col4 FROM tabella1" +
"WHERE col4 IN " +
"(SELECT col5 FROM tabella2" +
"WHERE col6='nome_col6' AND (CASE WHEN ISNUMERIC(col5)=1 THEN CAST (col5 AS INT) BETWEEN 30 AND 36 END))";

mi viene segnalato questo errore:
{"Incorrect syntax near the keyword 'BETWEEN'."}

sapreste dirmi perché?
Grazie