Dynamic SQL Works On Some Servers - Not Others

On some servers the below stored procedure runs fine. On others we get error message - 2147217900 Incorrect Syntax Near '___ ___' - which is from the line that starts with AND (ISNUMERIC(SUBSTRING etc
-----------------------------------------------------------------------------
IF @Specialty = 'None'
BEGIN -- No agent specialty selected.
EXEC('
TRUNCATE TABLE tblAgents
INSERT INTO tblAgents
SELECT DISTINCT c1.accountno, c1.recid, c1.company, c1.contact, c1.secr,
c1.address1, c1.address2, c1.city, c1.state, LEFT(UPPER(c1.zip),5) AS zip,
c1.phone1, c1.fax, '''' AS email, ''0'' AS dist, NULL AS lat, NULL AS long
FROM contact1 c1
WHERE c1.u_key1 LIKE ''' + @DsgntrAgnt + '''
AND (c1.u_key4 IN (' + @PrefDsgntrList + ')
AND (ISNUMERIC(SUBSTRING(c1.zip,1,5)) = 1) OR (c1.zip LIKE ''___ ___''))
')
END
--------------------------------------------------------------------------
Thanks

Doug