Nested /testforblock(s) command inside /execute does not work

When running this command,
"/execute @e[type=Sheep] ~ ~ ~ /testforblock ~ ~1 ~ iron_block"
in a command block, you get this output,
"[23:05:23] Failed to execute '/testforblock ~ ~1 ~ iron_block' as Sheep"

This does not happen in 14w21a/b