diff --git a/TypedSyntax/src/node.jl b/TypedSyntax/src/node.jl
index 5202a855..76fbe6b8 100644
--- a/TypedSyntax/src/node.jl
+++ b/TypedSyntax/src/node.jl
@@ -172,8 +172,8 @@ function map_signature!(sig::TypedSyntaxNode, slotnames::Vector{Symbol}, slottyp
         # Try to find an identifier for this arg
         arg, defaultval = argidentifier(arg)
         if arg !== nothing
-            name = arg.val::Symbol
-            idx = findfirst(==(name), slotnames)
+            name = arg.val
+            idx = name === nothing ? nothing : findfirst(==(name::Symbol), slotnames)
             if idx === nothing && isempty(slotarg) && slotnames[1] == Symbol("#self#")
                 # The first argument is the function name itself, and matches `#self#`
                 idx = 1
diff --git a/TypedSyntax/test/runtests.jl b/TypedSyntax/test/runtests.jl
index eaace65b..295238f0 100644
--- a/TypedSyntax/test/runtests.jl
+++ b/TypedSyntax/test/runtests.jl
@@ -693,6 +693,12 @@ include("test_module.jl")
     @test_nowarn str = sprint(tsn; context=:color=>false) do io, obj
         printstyled(io, obj; hide_type_stable=false)
     end
+
+    # issue 504
+    tsn = TypedSyntaxNode(TSN.A504{T, 1} where T, (Vector{Int},))
+    @test_nowarn str = sprint(tsn; context=:color=>false) do io, obj
+        printstyled(io, obj; hide_type_stable=false)
+    end
 end
 
 if parse(Bool, get(ENV, "CI", "false"))
diff --git a/TypedSyntax/test/test_module.jl b/TypedSyntax/test/test_module.jl
index 30493f08..d2d13bb4 100644
--- a/TypedSyntax/test/test_module.jl
+++ b/TypedSyntax/test/test_module.jl
@@ -249,6 +249,9 @@ end
 
 f597(var"#"::Int) = 1
 
+struct A504{T, N} end
+(A504{T, N} where T)(x::AbstractArray{S, N}) where {S, N} = x
+
 module Internal
 export helper
 helper(x) = x+1