diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ed4c0912..a778411a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,8 +16,8 @@ jobs:
       matrix:
         version:
           - '1.0'
-          - '1.5'
           - '1'
+          - 'nightly'
         os:  [ubuntu-latest, windows-latest, macos-latest] # adjust according to need, e.g. os: [ubuntu-latest] if testing only on linux
         arch:
           - x64
diff --git a/src/backwards_compatibility.jl b/src/backwards_compatibility.jl
index 7f525c50..412f0300 100644
--- a/src/backwards_compatibility.jl
+++ b/src/backwards_compatibility.jl
@@ -22,3 +22,16 @@ function read_array(f::JLDFile, dataspace::ReadDataspace,
     v = read_array(f, dataspace, rrv, data_length, filter_id, NULL_REFERENCE, attributes)
     String(v)
 end
+
+
+@static if VERSION < v"1.7"
+    # Location of `mutable` flag is moved from datatype to typename in julia v1.7
+    # Switch to using accessor function added in v1.7
+
+    # Borrowed from julia base
+    function ismutabletype(@nospecialize(t::Type))
+        t = unwrap_unionall(t)
+        # TODO: what to do for `Union`?
+        return isa(t, DataType) && t.mutable
+    end
+end
\ No newline at end of file
diff --git a/src/data/reconstructing_datatypes.jl b/src/data/reconstructing_datatypes.jl
index 97315c54..61148d55 100644
--- a/src/data/reconstructing_datatypes.jl
+++ b/src/data/reconstructing_datatypes.jl
@@ -514,7 +514,7 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
         # For bits types, we should always inline, because otherwise we'll just
         # pass a lot of crap around in registers
         push!(args, Expr(:meta, :inline))
-    elseif T.mutable
+    elseif ismutabletype(T)
         push!(args, quote
             obj = $(Expr(:new, T))
             track_weakref!(f, header_offset, obj)
@@ -527,7 +527,7 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
         rtype = types[i]
         odr = odrs[i]
 
-        if !T.mutable
+        if !ismutabletype(T)
             fsym = Symbol("field_", fn[i])
             push!(fsyms, fsym)
         end
@@ -538,12 +538,12 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
             # Type is not stored or single instance
             if T.types[i] == Union{}
                 # This cannot be defined
-                @assert !T.mutable
+                @assert !ismutabletype(T)
                 push!(args, Expr(:return, Expr(:new, T, fsyms[1:i-1]...)))
                 return blk
             else
                 newi = Expr(:new, T.types[i])
-                if T.mutable
+                if ismutabletype(T)
                     fni = QuoteNode(fn[i])
                     push!(args, :(setfield!(obj, $fni, $newi)))
                 else
@@ -557,7 +557,7 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
                         $(if T <: Tuple || i <= T.ninitialized
                             # Reference must always be initialized
                             :(throw(UndefinedFieldException(T,$(QuoteNode(fn[i])))))
-                        elseif T.mutable
+                        elseif ismutabletype(T)
                             # Reference could be uninitialized
                             :(return obj)
                         else
@@ -574,7 +574,7 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
             else
                 ttype = T.types[i]
                 fni = QuoteNode(fn[i])
-                if T.mutable
+                if ismutabletype(T)
                     push!(args, :(setfield!(obj, $fni,
                                             convert($ttype, jlconvert($rr, f, ptr+$offset, NULL_REFERENCE)::$rtype)::$ttype)))
                 else
@@ -584,7 +584,7 @@ jlconvert(::ReadRepresentation{Core.TypeofBottom,nothing}, f::JLDFile, ptr::Ptr,
         end
     end
 
-    push!(args, T.mutable ? (:obj) : T <: Tuple ? Expr(:tuple, fsyms...) : Expr(:new, T, fsyms...))
+    push!(args, ismutabletype(T) ? (:obj) : T <: Tuple ? Expr(:tuple, fsyms...) : Expr(:new, T, fsyms...))
 
     blk
 end
diff --git a/src/data/writing_datatypes.jl b/src/data/writing_datatypes.jl
index b2c16253..d4e59736 100644
--- a/src/data/writing_datatypes.jl
+++ b/src/data/writing_datatypes.jl
@@ -21,7 +21,7 @@ odr_sizeof(::ReadRepresentation{T,S}) where {T,S} = odr_sizeof(S)
     T = T::DataType
     T in encounteredtypes && return true
     push!(encounteredtypes, T)
-    (T.mutable || T <: Type) && return true
+    (ismutabletype(T) || T <: Type) && return true
     hasdata(T, encounteredtypes)
 end
 
@@ -68,7 +68,7 @@ fieldnames(@nospecialize x) = collect(Base.fieldnames(x))
         elseif isa(T, DataType)
             if isbitstype(T)
                 return :(odr(T))
-            elseif !T.mutable
+            elseif !ismutabletype(T)
                 return :(initialized ? odr(T) : RelOffset)
             end
         end
@@ -83,7 +83,7 @@ end
     if isconcretetype(T)
         if !hasfielddata(T)
             return nothing
-        elseif isbitstype(T) || (isa(initialized, Type{Type{Val{true}}}) && !T.mutable)
+        elseif isbitstype(T) || (isa(initialized, Type{Type{Val{true}}}) && !ismutabletype(T))
             return quote
                 @lookup_committed f T
                 $(if isempty(T.types)
@@ -511,7 +511,7 @@ end
 newstruct(T) = ccall(:jl_new_struct_uninit, Any, (Any,), T)
 
 function newstruct(T, fields)
-    if !T.mutable
+    if !ismutabletype(T)
         return ccall(:jl_new_structv, Any, (Any,Ptr{Cvoid},UInt32), T, fields, length(fields))
     else
         # Manual inline of newstruct! to work around bug