diff --git a/src/array.jl b/src/array.jl
index 8ed059e6..cef1e381 100644
--- a/src/array.jl
+++ b/src/array.jl
@@ -812,7 +812,7 @@ function _uniquerefs(A::CatArrOrSub{T}) where T
     res = similar(arefs, 0)
     nlevels = length(levels(A))
     maxunique = nlevels + (T >: Missing ? 1 : 0)
-    seen = fill(false, maxunique)
+    seen = fill(false, nlevels + 1) # always +1 for 0 (missing ref)
     @inbounds for ref in arefs
         if !seen[ref + 1]
             push!(res, ref)