diff --git a/src/abstractdataframe/iteration.jl b/src/abstractdataframe/iteration.jl
index 67f3d5287..c2484fca6 100644
--- a/src/abstractdataframe/iteration.jl
+++ b/src/abstractdataframe/iteration.jl
@@ -750,10 +750,10 @@ end
 
 # definition needed to avoid dispatch ambiguity
 Base.reduce(::typeof(vcat),
-    dfs::Union{SentinelArrays.ChainedVector{T,A} where {T::AbstractDataFrame, A<:AbstractVector{T}},
-        SentinelArrays.ChainedVector{T,A} where {T::DataFrame, A<:AbstractVector{T}},
-        SentinelArrays.ChainedVector{T,A} where {T::SubDataFrame, A<:AbstractVector{T}},
-        SentinelArrays.ChainedVector{T,A} where {T::Union{DataFrame, SubDataFrame}, A<:AbstractVector{T}}};
+    dfs::Union{SentinelArrays.ChainedVector{AbstractDataFrame,<:AbstractVector{AbstractDataFrame}},
+        SentinelArrays.ChainedVector{DataFrame,<:AbstractVector{DataFrame}},
+        SentinelArrays.ChainedVector{SubDataFrame,<:AbstractVector{SubDataFrame}},
+        SentinelArrays.ChainedVector{Union{DataFrame,SubDataFrame},<:AbstractVector{Union{DataFrame,SubDataFrame}}}};
     cols::Union{Symbol,AbstractVector{Symbol},
         AbstractVector{<:AbstractString}}=:setequal,
     source::Union{Nothing,SymbolOrString,