diff --git a/src/Turing.jl b/src/Turing.jl
index c172b864af..d6cea7b684 100644
--- a/src/Turing.jl
+++ b/src/Turing.jl
@@ -98,6 +98,10 @@ Robust initialization method for model parameters in Hamiltonian samplers.
 struct HamiltonianRobustInit <: AbstractSampler end
 struct SampleFromPrior <: AbstractSampler end
 
+# This can be removed when all `spl=nothing` is replaced with
+#   `spl=SampleFromPrior`
+const AnySampler = Union{Nothing, AbstractSampler}
+
 include("utilities/helper.jl")
 include("utilities/transform.jl")
 include("core/varinfo.jl")  # core internal variable container
diff --git a/src/core/compiler.jl b/src/core/compiler.jl
index 2f7211542c..20036cdf44 100644
--- a/src/core/compiler.jl
+++ b/src/core/compiler.jl
@@ -252,7 +252,7 @@ macro model(fexpr)
         :kwargs => [],
         :args => [
             :(vi::Turing.VarInfo),
-            :(sampler::Turing.AbstractSampler)
+            :(sampler::Turing.AnySampler)
         ],
         :body => body
     )
@@ -272,7 +272,7 @@ macro model(fexpr)
     alias2 = MacroTools.combinedef(
                 Dict(
                     :name => compiler[:closure_name],
-                    :args => [:(sampler::Turing.AbstractSampler)],
+                    :args => [:(sampler::Turing.AnySampler)],
                     :kwargs => [],
                     :body => :(return $(compiler[:closure_name])(Turing.VarInfo(), Turing.SampleFromPrior()))
 
diff --git a/src/samplers/sampler.jl b/src/samplers/sampler.jl
index 0d0801c1d8..51f398f255 100644
--- a/src/samplers/sampler.jl
+++ b/src/samplers/sampler.jl
@@ -34,10 +34,13 @@ observe(spl::Sampler, weight::Float64) =
 error("Turing.observe: unmanaged inference algorithm: $(typeof(spl))")
 
 ## Default definitions for assume, observe, when sampler = nothing.
+##  Note: `A<:Union{Nothing, SampleFromPrior, HamiltonianRobustInit}` can be
+##   simplified into `A<:Union{SampleFromPrior, HamiltonianRobustInit}` when
+##   all `spl=nothing` is replaced with `spl=SampleFromPrior`.
 function assume(spl::A,
     dist::Distribution,
     vn::VarName,
-    vi::VarInfo) where {A<:Union{HamiltonianRobustInit, SampleFromPrior, Nothing}}
+    vi::VarInfo) where {A<:Union{Nothing, SampleFromPrior, HamiltonianRobustInit}}
 
     if haskey(vi, vn)
         r = vi[vn]
@@ -57,7 +60,7 @@ function assume(spl::A,
     dists::Vector{T},
     vn::VarName,
     var::Any,
-    vi::VarInfo) where {T<:Distribution, A<:Union{HamiltonianRobustInit, SampleFromPrior, Nothing}}
+    vi::VarInfo) where {T<:Distribution, A<:Union{Nothing, SampleFromPrior, HamiltonianRobustInit}}
 
     @assert length(dists) == 1 "Turing.assume only support vectorizing i.i.d distribution"
     dist = dists[1]
@@ -103,7 +106,7 @@ end
 function observe(spl::A,
     dist::Distribution,
     value::Any,
-    vi::VarInfo) where {A<:Union{HamiltonianRobustInit, SampleFromPrior, Nothing}}
+    vi::VarInfo) where {A<:Union{Nothing, SampleFromPrior, HamiltonianRobustInit}}
 
     vi.num_produce += 1
     @debug "dist = $dist"
@@ -117,7 +120,7 @@ end
 function observe(spl::A,
     dists::Vector{T},
     value::Any,
-    vi::VarInfo) where {T<:Distribution, A<:Union{HamiltonianRobustInit, SampleFromPrior, Nothing}}
+    vi::VarInfo) where {T<:Distribution, A<:Union{Nothing, SampleFromPrior, HamiltonianRobustInit}}
 
     @assert length(dists) == 1 "Turing.observe only support vectorizing i.i.d distribution"
     dist = dists[1]
diff --git a/test/compiler.jl/model_macro.jl b/test/compiler.jl/model_macro.jl
index c39789f807..a44d2ac887 100644
--- a/test/compiler.jl/model_macro.jl
+++ b/test/compiler.jl/model_macro.jl
@@ -27,15 +27,15 @@ alias1 = Dict(
             :name => :testmodel_comp_model,
             :args => [:(vi::Turing.VarInfo)],
             :kwargs => [],
-            :body => :(return testmodel_comp_model(vi, nothing))
+            :body => :(return testmodel_comp_model(vi, Turing.SampleFromPrior()))
            )
 @test c[:alias1] == MacroTools.combinedef(alias1)
 
 alias2 = Dict(
             :name => :testmodel_comp_model,
-            :args => [:(sampler::Turing.AbstractSampler)],
+            :args => [:(sampler::Turing.AnySampler)],
             :kwargs => [],
-            :body => :(return testmodel_comp_model(Turing.VarInfo(), nothing))
+            :body => :(return testmodel_comp_model(Turing.VarInfo(), Turing.SampleFromPrior()))
            )
 @test c[:alias2] == MacroTools.combinedef(alias2)
 
@@ -43,7 +43,7 @@ alias3 = Dict(
             :name => :testmodel_comp_model,
             :args => [],
             :kwargs => [],
-            :body => :(return testmodel_comp_model(Turing.VarInfo(), nothing))
+            :body => :(return testmodel_comp_model(Turing.VarInfo(), Turing.SampleFromPrior()))
            )
 @test c[:alias3] == MacroTools.combinedef(alias3)
 @test length(c[:closure].args[2].args[2].args) == 6