diff --git a/src/plfa/part2/Properties.lagda.md b/src/plfa/part2/Properties.lagda.md
index af0485ec..3ab89019 100644
--- a/src/plfa/part2/Properties.lagda.md
+++ b/src/plfa/part2/Properties.lagda.md
@@ -807,7 +807,7 @@ preserve ((⊢ƛ ⊢N) · ⊢V)          (β-ƛ VV)         =  subst ⊢V ⊢N
 preserve ⊢zero                   ()
 preserve (⊢suc ⊢M)               (ξ-suc M—→M′)    =  ⊢suc (preserve ⊢M M—→M′)
 preserve (⊢case ⊢L ⊢M ⊢N)        (ξ-case L—→L′)   =  ⊢case (preserve ⊢L L—→L′) ⊢M ⊢N
-preserve (⊢case ⊢zero ⊢M ⊢N)     β-zero           =  ⊢M
+preserve (⊢case ⊢zero ⊢M ⊢N)     (β-zero)         =  ⊢M
 preserve (⊢case (⊢suc ⊢V) ⊢M ⊢N) (β-suc VV)       =  subst ⊢V ⊢N
 preserve (⊢μ ⊢M)                 (β-μ)            =  subst (⊢μ ⊢M) ⊢M
 ```