diff --git a/src/io.jl b/src/io.jl
index c3ce35d..2b6d515 100644
--- a/src/io.jl
+++ b/src/io.jl
@@ -235,12 +235,21 @@ function _evenlydistribute!(t, wrkrs)
     end
 end
 
+_ismutabletype = if VERSION < v"1.7"
+    function ismutabletype(@nospecialize(t::Type))
+        t = unwrap_unionall(t)
+        return isa(t, DataType) && t.name.flags & 0x2 == 0x2
+    end
+else
+    Base.ismutabletype
+end
+
 deserialize(io::AbstractSerializer, DT::Type{DNDSparse{K,V}}) where {K,V} = _deser(io, DT)
 deserialize(io::AbstractSerializer, DT::Type{DIndexedTable{T,K}}) where {T,K} = _deser(io, DT)
 function _deser(io::AbstractSerializer, t)
     nf = fieldcount(t)
     x = ccall(:jl_new_struct_uninit, Any, (Any,), t)
-    t.mutable && Serialization.deserialize_cycle(io, x)
+    _ismutabletype(t) && Serialization.deserialize_cycle(io, x)
     for i in 1:nf
         tag = Int32(read(io.io, UInt8)::UInt8)
         if tag != Serialization.UNDEFREF_TAG