SQL & PL/SQL

I have the following script:

SELECT ar.ruta,dto.descripcion Departamento,mun.descripcion Municipio,
SUM(DECODE(ar.diavisita,'1',COUNT(ar.no_cliente),0))lunes,
SUM(DECODE(ar.diavisita,'2',COUNT(ar.no_cliente),0))Martes,
SUM(DECODE(ar.diavisita,'3',COUNT(ar.no_cliente),0))Miercoles,
SUM(DECODE(ar.diavisita,'4',COUNT(ar.no_cliente),0))Jueves,
SUM(DECODE(ar.diavisita,'5',COUNT(ar.no_cliente),0))Viernes,
SUM(DECODE(ar.diavisita,'6',COUNT(ar.no_cliente),0))Sabado
FROM arccar ar, arccpro dto, arcccan mun, arccmc mc
WHERE (mc.no_cia = ar.no_cia
AND mc.centro = ar.centro
AND mc.grupo = ar.grupo
AND mc.no_cliente = ar.no_cliente)
AND (dto.pais = mc.pais
AND dto.zona = mc.zona
AND dto.provincia = mc.provincia)
AND (mun.pais(+) = mc.pais
AND mun.provincia(+) = mc.provincia
AND mun.canton(+) = mc.canton)
AND mc.activo = 'S'
AND ar.ruta = '311'
AND ar.semana = '2'
GROUP BY ar.ruta,dto.descripcion,mun.descripcion,ar.diavisita

...when I execute the query in PL/SQL Developer I get this error, Error ORA-00937: not a single-group group function.

Can somebody help me.