diff --git a/src/arg_desc.jl b/src/arg_desc.jl
index afccd2e6..eecf39db 100644
--- a/src/arg_desc.jl
+++ b/src/arg_desc.jl
@@ -27,8 +27,8 @@ const _arg_desc = KW(
 :x                 	=> "Various. Input data. First Dimension",
 :y                 	=> "Various. Input data. Second Dimension",
 :z                 	=> "Various. Input data. Third Dimension. May be wrapped by a `Surface` for surface and heatmap types.",
-:marker_z          	=> "AbstractVector, Function `f(x,y,z) -> z_value`, or nothing. z-values for each series data point, which correspond to the color to be used from a markercolor gradient.",
-:line_z          	=> "AbstractVector, Function `f(x,y,z) -> z_value`, or nothing. z-values for each series line segment, which correspond to the color to be used from a linecolor gradient.  Note that for N points, only the first N-1 values are used (one per line-segment).",
+:marker_z          	=> "AbstractVector, Function `f(x,y,z) -> z_value`, or Function `f(x,y) -> z_value`, or nothing. z-values for each series data point, which correspond to the color to be used from a markercolor gradient.",
+:line_z          	=> "AbstractVector, Function `f(x,y,z) -> z_value`, or Function `f(x,y) -> z_value`, or nothing. z-values for each series line segment, which correspond to the color to be used from a linecolor gradient.  Note that for N points, only the first N-1 values are used (one per line-segment).",
 :fill_z           => "Matrix{Float64} of the same size as z matrix, which specifies the color of the 3D surface; the default value is `nothing`.",
 :levels            	=> "Integer, NTuple{2,Integer}. Number of levels (or x-levels/y-levels) for a contour type.",
 :orientation       	=> "Symbol.  Horizontal or vertical orientation for bar types.  Values `:h`, `:hor`, `:horizontal` correspond to horizontal (sideways, anchored to y-axis), and `:v`, `:vert`, and `:vertical` correspond to vertical (the default).",
diff --git a/src/pipeline.jl b/src/pipeline.jl
index 110a29bc..5c1772db 100644
--- a/src/pipeline.jl
+++ b/src/pipeline.jl
@@ -117,12 +117,12 @@ function _preprocess_userrecipe(kw::KW)
     # map marker_z if it's a Function
     if isa(get(kw, :marker_z, nothing), Function)
         # TODO: should this take y and/or z as arguments?
-        kw[:marker_z] = isa(kw[:z], Void) ? map(kw[:marker_z], kw[:x], kw[:y]) : map(kw[:marker_z], kw[:x], kw[:y], kw[:z])
+        kw[:marker_z] = isa(kw[:z], Nothing) ? map(kw[:marker_z], kw[:x], kw[:y]) : map(kw[:marker_z], kw[:x], kw[:y], kw[:z])
     end
 
     # map line_z if it's a Function
     if isa(get(kw, :line_z, nothing), Function)
-        kw[:line_z] = map(kw[:line_z], kw[:x], kw[:y], kw[:z])
+        kw[:line_z] = isa(kw[:z], Nothing) ? map(kw[:line_z], kw[:x], kw[:y]) : map(kw[:line_z], kw[:x], kw[:y], kw[:z])
     end
 
     # convert a ribbon into a fillrange