diff --git a/test/runtests.jl b/test/runtests.jl
index 6cfebd8..7bdbad8 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -13,6 +13,15 @@ end
 Base.show(io::IO, m::M, x::DummyDisplayable{M}) where {M} = true
 Base.show(io::IO, m::M, x::DummyDisplayable{M}) where {M} = print(io, x.data)
 
+# These are normally defined in Vega.jl and VegaLite.jl, but because we don't use
+# those packages here, we redefine them just for the tests
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vega.v3+json")}) = true
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vega.v4+json")}) = true
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vega.v5+json")}) = true
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vegalite.v2+json")}) = true
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vegalite.v3+json")}) = true
+Base.Multimedia.istextmime(::MIME{Symbol("application/vnd.vegalite.v4+json")}) = true
+
 include("construct_vega_specs.jl")
 
 @testset "ElectronDisplay" begin