Agda 2.5.1 can't parse _+_

[langston-barrett]

When checking the definition of _+_, I get the following error:

/home/siddharthist/code/agda/LearnYouAn.agda:12,1-13
Could not parse the left-hand side (suc n) + n'
Operators used in the grammar:
  + (infix operator, level 20) [_+_ (/home/siddharthist/code/agda/LearnYouAn.agda:9,1-4)]
(the treatment of operators was changed in Agda 2.5.1, so code that
used to parse may have to be changed)
when scope checking the left-hand side (suc n) + n' in the
definition of _+_

Here's the code:

_+_ : ℕ → ℕ → ℕ
zero + zero = zero
zero + n = n
(suc n) + n' = suc (n + n')