Fix ambiguity

Author: jipolanco

src/Recombinations/matrices.jl

@@ -149,8 +149,8 @@ end
 # Default element type of recombination matrix.
 # In some specific cases we can use Bool...
 _default_eltype(::Type{T}, ::BoundaryCondition) where {T <: AbstractFloat} = T
-_default_eltype(::Type{T}, ::Derivative{0}) where {T} = Bool  # Dirichlet BCs
-_default_eltype(::Type{T}, ::Derivative{1}) where {T} = Bool  # Neumann BCs
+_default_eltype(::Type{T}, ::Derivative{0}) where {T <: AbstractFloat} = Bool  # Dirichlet BCs
+_default_eltype(::Type{T}, ::Derivative{1}) where {T <: AbstractFloat} = Bool  # Neumann BCs
 _default_eltype(::Type{T}, ::Vararg{AbstractDifferentialOp}) where {T <: AbstractFloat} = T
 
 # Case (D(0), D(1), D(2), ...)