diff --git a/Project.toml b/Project.toml
index eac9803c..a15ec484 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "DynamicExpressions"
 uuid = "a40a106e-89c9-4ca8-8020-a735e8728b6b"
 authors = ["MilesCranmer <miles.cranmer@gmail.com>"]
-version = "0.18.0"
+version = "0.18.1"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
diff --git a/ext/DynamicExpressionsSymbolicUtilsExt.jl b/ext/DynamicExpressionsSymbolicUtilsExt.jl
index 28c3b12a..5f2dd267 100644
--- a/ext/DynamicExpressionsSymbolicUtilsExt.jl
+++ b/ext/DynamicExpressionsSymbolicUtilsExt.jl
@@ -111,7 +111,7 @@ end
 function Base.convert(
     ::typeof(SymbolicUtils.Symbolic),
     tree::Union{AbstractExpression,AbstractExpressionNode},
-    operators::AbstractOperatorEnum;
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
     variable_names::Union{Array{String,1},Nothing}=nothing,
     index_functions::Bool=false,
     # Deprecated:
@@ -119,7 +119,10 @@ function Base.convert(
 )
     variable_names = deprecate_varmap(variable_names, varMap, :convert)
     return node_to_symbolic(
-        tree, operators; variable_names=variable_names, index_functions=index_functions
+        tree,
+        get_operators(tree, operators);
+        variable_names=variable_names,
+        index_functions=index_functions,
     )
 end
 
diff --git a/src/Expression.jl b/src/Expression.jl
index 986b85de..9d1e0e3f 100644
--- a/src/Expression.jl
+++ b/src/Expression.jl
@@ -101,7 +101,9 @@ or `cur_operators` if it is not `nothing`. If left as default,
 it requires `cur_operators` to not be `nothing`.
 `cur_operators` would typically be an `OperatorEnum`.
 """
-function get_operators(ex::AbstractExpression, operators)
+function get_operators(
+    ex::AbstractExpression, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+)
     return error("`get_operators` function must be implemented for $(typeof(ex)) types.")
 end
 
@@ -110,7 +112,10 @@ end
 
 The same as `operators`, but for variable names.
 """
-function get_variable_names(ex::AbstractExpression, variable_names)
+function get_variable_names(
+    ex::AbstractExpression,
+    variable_names::Union{Nothing,AbstractVector{<:AbstractString}}=nothing,
+)
     return error(
         "`get_variable_names` function must be implemented for $(typeof(ex)) types."
     )
@@ -179,10 +184,23 @@ function preserve_sharing(::Union{E,Type{E}}) where {T,N,E<:AbstractExpression{T
     return preserve_sharing(N)
 end
 
-function get_operators(ex::Expression, operators=nothing)
+function get_operators(
+    tree::AbstractExpressionNode, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+)
+    if operators === nothing
+        throw(ArgumentError("`operators` must be provided for $(typeof(tree)) types."))
+    else
+        return operators
+    end
+end
+function get_operators(
+    ex::Expression, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+)
     return operators === nothing ? ex.metadata.operators : operators
 end
-function get_variable_names(ex::Expression, variable_names=nothing)
+function get_variable_names(
+    ex::Expression, variable_names::Union{Nothing,AbstractVector{<:AbstractString}}=nothing
+)
     return variable_names === nothing ? ex.metadata.variable_names : variable_names
 end
 function get_tree(ex::Expression)
@@ -249,7 +267,10 @@ end
 import ..StringsModule: string_tree, print_tree
 
 function string_tree(
-    ex::AbstractExpression, operators=nothing; variable_names=nothing, kws...
+    ex::AbstractExpression,
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
+    variable_names=nothing,
+    kws...,
 )
     return string_tree(
         get_tree(ex),
@@ -260,7 +281,11 @@ function string_tree(
 end
 for io in ((), (:(io::IO),))
     @eval function print_tree(
-        $(io...), ex::AbstractExpression, operators=nothing; variable_names=nothing, kws...
+        $(io...),
+        ex::AbstractExpression,
+        operators::Union{AbstractOperatorEnum,Nothing}=nothing;
+        variable_names=nothing,
+        kws...,
     )
         return println($(io...), string_tree(ex, operators; variable_names, kws...))
     end
@@ -283,7 +308,9 @@ function max_feature(ex::AbstractExpression)
     )
 end
 
-function _validate_input(ex::AbstractExpression, X, operators)
+function _validate_input(
+    ex::AbstractExpression, X, operators::Union{AbstractOperatorEnum,Nothing}
+)
     if get_operators(ex, operators) isa OperatorEnum
         @assert X isa AbstractMatrix
         @assert max_feature(ex) <= size(X, 1)
@@ -292,7 +319,10 @@ function _validate_input(ex::AbstractExpression, X, operators)
 end
 
 function eval_tree_array(
-    ex::AbstractExpression, cX::AbstractMatrix, operators=nothing; kws...
+    ex::AbstractExpression,
+    cX::AbstractMatrix,
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
+    kws...,
 )
     _validate_input(ex, cX, operators)
     return eval_tree_array(get_tree(ex), cX, get_operators(ex, operators); kws...)
@@ -305,7 +335,10 @@ import ..EvaluateDerivativeModule: eval_grad_tree_array
 #  - differentiable_eval_tree_array
 
 function eval_grad_tree_array(
-    ex::AbstractExpression, cX::AbstractMatrix, operators=nothing; kws...
+    ex::AbstractExpression,
+    cX::AbstractMatrix,
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
+    kws...,
 )
     _validate_input(ex, cX, operators)
     return eval_grad_tree_array(get_tree(ex), cX, get_operators(ex, operators); kws...)
@@ -319,14 +352,16 @@ end
 function _grad_evaluator(
     ex::AbstractExpression,
     cX::AbstractMatrix,
-    operators=nothing;
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
     variable=Val(true),
     kws...,
 )
     _validate_input(ex, cX, operators)
     return _grad_evaluator(get_tree(ex), cX, get_operators(ex, operators); variable, kws...)
 end
-function (ex::AbstractExpression)(X, operators=nothing; kws...)
+function (ex::AbstractExpression)(
+    X, operators::Union{AbstractOperatorEnum,Nothing}=nothing; kws...
+)
     _validate_input(ex, X, operators)
     return get_tree(ex)(X, get_operators(ex, operators); kws...)
 end
diff --git a/src/ParametricExpression.jl b/src/ParametricExpression.jl
index d4ecac10..09ce8aa3 100644
--- a/src/ParametricExpression.jl
+++ b/src/ParametricExpression.jl
@@ -2,6 +2,7 @@ module ParametricExpressionModule
 
 using DispatchDoctor: @stable, @unstable
 
+using ..OperatorEnumModule: AbstractOperatorEnum
 using ..NodeModule: AbstractExpressionNode, Node, tree_mapreduce
 using ..ExpressionModule: AbstractExpression, Metadata
 
@@ -70,7 +71,7 @@ struct ParametricExpression{
 end
 function ParametricExpression(
     tree::ParametricNode{T1};
-    operators,
+    operators::Union{AbstractOperatorEnum,Nothing},
     variable_names,
     parameters::AbstractMatrix{T2},
     parameter_names,
@@ -141,10 +142,15 @@ end
 get_contents(ex::ParametricExpression) = ex.tree
 get_metadata(ex::ParametricExpression) = ex.metadata
 get_tree(ex::ParametricExpression) = ex.tree
-function get_operators(ex::ParametricExpression, operators=nothing)
+function get_operators(
+    ex::ParametricExpression, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+)
     return operators === nothing ? ex.metadata.operators : operators
 end
-function get_variable_names(ex::ParametricExpression, variable_names=nothing)
+function get_variable_names(
+    ex::ParametricExpression,
+    variable_names::Union{Nothing,AbstractVector{<:AbstractString}}=nothing,
+)
     return variable_names === nothing ? ex.metadata.variable_names : variable_names
 end
 @inline _copy_with_nothing(x) = copy(x)
@@ -232,15 +238,18 @@ function Base.convert(::Type{Node}, ex::ParametricExpression{T}) where {T}
     )
 end
 #! format: off
-function (ex::ParametricExpression)(X::AbstractMatrix, operators=nothing; kws...)
+function (ex::ParametricExpression)(X::AbstractMatrix, operators::Union{AbstractOperatorEnum,Nothing}=nothing; kws...)
     return eval_tree_array(ex, X, operators; kws...)  # Will error
 end
-function eval_tree_array(::ParametricExpression{T}, ::AbstractMatrix{T}, operators=nothing; kws...) where {T}
+function eval_tree_array(::ParametricExpression{T}, ::AbstractMatrix{T}, operators::Union{AbstractOperatorEnum,Nothing}=nothing; kws...) where {T}
     return error("Incorrect call. You must pass the `classes::Vector` argument when calling `eval_tree_array`.")
 end
 #! format: on
 function (ex::ParametricExpression)(
-    X::AbstractMatrix{T}, classes::AbstractVector{<:Integer}, operators=nothing; kws...
+    X::AbstractMatrix{T},
+    classes::AbstractVector{<:Integer},
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
+    kws...,
 ) where {T}
     (output, flag) = eval_tree_array(ex, X, classes, operators; kws...)  # Will error
     if !flag
@@ -252,7 +261,7 @@ function eval_tree_array(
     ex::ParametricExpression{T},
     X::AbstractMatrix{T},
     classes::AbstractVector{<:Integer},
-    operators=nothing;
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
     kws...,
 ) where {T}
     @assert length(classes) == size(X, 2)
@@ -270,7 +279,7 @@ function eval_tree_array(
 end
 function string_tree(
     ex::ParametricExpression,
-    operators=nothing;
+    operators::Union{AbstractOperatorEnum,Nothing}=nothing;
     variable_names=nothing,
     display_variable_names=nothing,
     X_sym_units=nothing,
diff --git a/test/test_expressions.jl b/test/test_expressions.jl
index 75fede26..1fb9470d 100644
--- a/test/test_expressions.jl
+++ b/test/test_expressions.jl
@@ -226,9 +226,13 @@ end
 
 @testitem "Miscellaneous expression calls" begin
     using DynamicExpressions
+    using DynamicExpressions: get_tree, get_operators
 
     ex = @parse_expression(x1 + 1.5, binary_operators = [+], variable_names = ["x1"])
     @test DynamicExpressions.ExpressionModule.node_type(ex) <: Node
 
     @test !isempty(ex)
+
+    tree = get_tree(ex)
+    @test_throws ArgumentError get_operators(tree, nothing)
 end
diff --git a/test/test_multi_expression.jl b/test/test_multi_expression.jl
index 56ee6d0e..e049aebd 100644
--- a/test/test_multi_expression.jl
+++ b/test/test_multi_expression.jl
@@ -87,10 +87,15 @@
         )::Expression{T,N}
         return fused_expression.tree
     end
-    function DE.get_operators(ex::MultiScalarExpression, operators=nothing)
+    function DE.get_operators(
+        ex::MultiScalarExpression, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+    )
         return operators === nothing ? ex.metadata.operators : operators
     end
-    function DE.get_variable_names(ex::MultiScalarExpression, variable_names=nothing)
+    function DE.get_variable_names(
+        ex::MultiScalarExpression,
+        variable_names::Union{Nothing,AbstractVector{<:AbstractString}}=nothing,
+    )
         return variable_names === nothing ? ex.metadata.variable_names : variable_names
     end
     function Base.copy(ex::MultiScalarExpression)