Testforblock can't detect signs with a '=' in them

The command I'm trying to execute:

execute @e[tag=CC] ~ ~ ~ testforblock ~ ~ ~ wall_sign 4 {Text4:"{\"text\":\"\"}",Text3:"{\"text\":\"\"}",Text2:"{\"text\":\"\"}",Text1:"{\"text\":\"=\"}"}

It works with everthing except '='. This exact command was used to detect over 150 different lines of text on sign. All of them work except this one.

Error message in command block:

[16:42:58] Failed to execute 'testforblock ~ ~ ~ wall_sign 4 {Text4:"{\"text\":\"\"}",Text3:"{\"text\":\"\"}",Text2:"{\"text\":\"\"}",Text1:"{\"text\":\"=\"}"}' as Armor Stand