diff --git a/src/common.jl b/src/common.jl
index 2bdba7c4..f753eabd 100644
--- a/src/common.jl
+++ b/src/common.jl
@@ -851,6 +851,7 @@ function Base.:+(p::P, c::S) where {T,X, P<:AbstractPolynomial{T,X}, S}
     q + R(c)
 end
 
+
 # polynomial + polynomial when different types
 function Base.:+(p::P, q::Q) where {T,X,P <: AbstractPolynomial{T,X}, S,Y,Q <: AbstractPolynomial{S,Y}}
     isconstant(p) && return constantterm(p) + q
@@ -944,6 +945,12 @@ function scalar_mult(c::S, p::P) where {S, T, X, P<:AbstractPolynomial{T, X}}
     𝐏([c * pᵢ for pᵢ ∈ coeffs(p)])
 end
 
+scalar_mult(c::S, p::Union{P, R}) where {
+    S<: AbstractPolynomial,
+    T, X,
+    P<:AbstractPolynomial{T, X},
+    R <:AbstractPolynomial{T}
+} = throw(DomainError()) # avoid ambiguity, issue #435
 
 function Base.:/(p::P, c::S) where {P <: AbstractPolynomial,S}
     _convert(p, coeffs(p) ./ c)
@@ -1082,6 +1089,11 @@ function Base.isapprox(p1::AbstractPolynomial{T},
     return isapprox(p1, _convert(p1, [n]))
 end
 
+Base.isapprox(::AbstractPolynomial{T}, ::Missing, args...; kwargs...) where T =
+    missing
+Base.isapprox(::Missing, ::AbstractPolynomial{T}, args...; kwargs...) where T =
+    missing
+
 Base.isapprox(n::S,
     p1::AbstractPolynomial{T};
     rtol::Real = (Base.rtoldefault(T, S, 0)),
diff --git a/src/polynomials/ChebyshevT.jl b/src/polynomials/ChebyshevT.jl
index c80d688d..52ff818a 100644
--- a/src/polynomials/ChebyshevT.jl
+++ b/src/polynomials/ChebyshevT.jl
@@ -222,7 +222,8 @@ function Base.:+(p::ChebyshevT{T,X}, c::S) where {T,X, S<:Number}
     cs[1] += c
     ChebyshevT{R,X}(cs)
 end
-function Base.:+(p::P, c::T) where {T,X,P<:ChebyshevT{T,X}}
+
+function Base.:+(p::P, c::T) where {T <: Number,X,P<:ChebyshevT{T,X}}
     cs = collect(T, values(p))
     cs[1] += c
     P(cs)
diff --git a/src/polynomials/ImmutablePolynomial.jl b/src/polynomials/ImmutablePolynomial.jl
index c9c68f51..874268e9 100644
--- a/src/polynomials/ImmutablePolynomial.jl
+++ b/src/polynomials/ImmutablePolynomial.jl
@@ -203,7 +203,7 @@ function scalar_mult(c::S, p::ImmutablePolynomial{T,X,N}) where {T, X,N, S <: Nu
     return ImmutablePolynomial(cs, X)
 end
 
-function Base.:/(p::ImmutablePolynomial{T,X,N}, c::S) where {T,X,N,S}
+function Base.:/(p::ImmutablePolynomial{T,X,N}, c::S) where {T,X,N,S<:Number}
     R = eltype(one(T)/one(S))
     P = ImmutablePolynomial{R,X}
     (N == 0  || isinf(c)) && return zero(P)
diff --git a/src/polynomials/LaurentPolynomial.jl b/src/polynomials/LaurentPolynomial.jl
index 2137fac4..e009280a 100644
--- a/src/polynomials/LaurentPolynomial.jl
+++ b/src/polynomials/LaurentPolynomial.jl
@@ -151,10 +151,15 @@ function Base.convert(::Type{P}, q::StandardBasisPolynomial{S}) where {P <:Laure
      ⟒(P){T,X}([q[i] for i in eachindex(q)], firstindex(q))
  end
 
-function Base.convert(::Type{P}, q::AbstractPolynomial) where {P <:LaurentPolynomial}
+function Base.convert(::Type{P}, q::AbstractPolynomial{T,X}) where {T,X,P <:LaurentPolynomial}
     convert(P, convert(Polynomial, q))
 end
 
+# Ambiguity, issue #435
+function Base.convert(𝑷::Type{P}, p::ArnoldiFit{T, M, X}) where {P<:LaurentPolynomial, T, M, X}
+    convert(𝑷, convert(Polynomial, p))
+end
+
 ##
 ## generic functions
 ##
diff --git a/src/polynomials/factored_polynomial.jl b/src/polynomials/factored_polynomial.jl
index 38915b4a..7110e9b9 100644
--- a/src/polynomials/factored_polynomial.jl
+++ b/src/polynomials/factored_polynomial.jl
@@ -273,7 +273,7 @@ function scalar_mult(p::P, c::S) where {S<:Number, T, X, P <: FactoredPolynomial
 end
 
 # scalar division
-function Base.:/(p::P, c::S) where {S, T, X, P <: FactoredPolynomial{T, X}}
+function Base.:/(p::P, c::S) where {S<:Number, T, X, P <: FactoredPolynomial{T, X}}
     p * (1/c)
 end
 
diff --git a/src/polynomials/mutable-arithmetics.jl b/src/polynomials/mutable-arithmetics.jl
index c39527b9..0e106d9c 100644
--- a/src/polynomials/mutable-arithmetics.jl
+++ b/src/polynomials/mutable-arithmetics.jl
@@ -65,3 +65,7 @@ macro register_mutable_arithmetic(name)
         end
     end
 end
+
+## Ambiguities. Issue #435
+Base.:+(p::P, ::MutableArithmetics.Zero) where {T, X, P<:AbstractPolynomial{T, X}} = p
+Base.:+(p::P, ::T) where {T<:MutableArithmetics.Zero, P<:StandardBasisPolynomial{T}} = p
diff --git a/src/polynomials/standard-basis.jl b/src/polynomials/standard-basis.jl
index 8bcd2e58..1ffaa70d 100644
--- a/src/polynomials/standard-basis.jl
+++ b/src/polynomials/standard-basis.jl
@@ -644,7 +644,7 @@ evalpoly(x, p::ArnoldiFit) = polyvalA(p.coeffs, p.H, x)
 
 fit(::Type{ArnoldiFit}, x::AbstractVector{T}, y::AbstractVector{T}, deg::Int=length(x)-1;  var=:x, kwargs...) where{T} = polyfitA(x, y, deg; var=var)
 
-Base.convert(::Type{P}, p::ArnoldiFit) where {P <: AbstractPolynomial} = p(variable(P,indeterminate(p)))
+Base.convert(::Type{P}, p::ArnoldiFit{T,M,X}) where {P <: AbstractPolynomial,T,M,X} = p(variable(P,indeterminate(p)))
 
 
 
diff --git a/src/rational-functions/common.jl b/src/rational-functions/common.jl
index 580400ab..f92cdc80 100644
--- a/src/rational-functions/common.jl
+++ b/src/rational-functions/common.jl
@@ -298,6 +298,7 @@ Base.:+(p::Number, q::AbstractRationalFunction) = q + p
 Base.:+(p::AbstractRationalFunction,  q::Number) = p + q*one(p)
 Base.:+(p::AbstractPolynomial, q::AbstractRationalFunction) = q + p
 Base.:+(p::AbstractRationalFunction,  q::AbstractPolynomial) = p + (q//one(q))
+Base.:+(p::P,  q::T) where {T<: AbstractRationalFunction, P<:StandardBasisPolynomial{T}} = throw(DomainError()) # avoid ambiguity (issue #435.
 Base.:+(p::AbstractRationalFunction, q::AbstractRationalFunction) = sum(promote(p,q))
 # type should implement this
 function Base.:+(p::R, q::R) where {T,X,P,R <: AbstractRationalFunction{T,X,P}}