diff --git a/Project.toml b/Project.toml
index a8378ec..2ad0ac3 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "SpineInterface"
 uuid = "0cda1612-498a-11e9-3c92-77fa82595a4f"
 authors = ["Spine Project consortium <spine_info@vtt.fi>"]
-version = "0.13.4"
+version = "0.13.5"
 
 [deps]
 DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
diff --git a/src/api/core.jl b/src/api/core.jl
index 3f3155a..a67e3c8 100644
--- a/src/api/core.jl
+++ b/src/api/core.jl
@@ -163,9 +163,11 @@ _find_rels(rc, rows) = (rc.relationships[row] for row in rows)
 _find_rels(rc, ::Anything) = rc.relationships
 
 function _find_rows(rc; kwargs...)
-    memoized_rows = get!(rc.row_map, rc.name, Dict())
-    get!(memoized_rows, kwargs) do
-        _do_find_rows(rc; kwargs...)
+    lock(rc.row_map_lock) do
+        memoized_rows = get!(rc.row_map, rc.name, Dict())
+        get!(memoized_rows, kwargs) do
+            _do_find_rows(rc; kwargs...)
+        end
     end
 end
 
@@ -714,7 +716,7 @@ function with_env(f::Function, env::Symbol)
     prev_env = _active_env()
     _activate_env(env)
     try
-        f()
+        return f()
     finally
         _activate_env(prev_env)
     end
diff --git a/src/types.jl b/src/types.jl
index b14e921..1af7990 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -72,12 +72,13 @@ struct TimeSlice
     id::UInt64
     actual_duration::Union{Dates.CompoundPeriod,Period}
     updates::OrderedDict
+    updates_lock::ReentrantLock
     function TimeSlice(start, end_, duration, blocks)
         start > end_ && error("out of order")
         blocks = isempty(blocks) ? () : Tuple(sort(collect(blocks)))
         id = objectid((start, end_, duration, blocks))
         actual_duration = canonicalize(end_ - start)
-        new(Ref(start), Ref(end_), duration, blocks, id, actual_duration, OrderedDict())
+        new(Ref(start), Ref(end_), duration, blocks, id, actual_duration, OrderedDict(), ReentrantLock())
     end
 end
 
@@ -119,11 +120,21 @@ struct _RelationshipClass
     parameter_values::Dict{ObjectTupleLike,Dict{Symbol,ParameterValue}}
     parameter_defaults::Dict{Symbol,ParameterValue}
     row_map::Dict
+    row_map_lock::ReentrantLock
     _split_kwargs::Ref{Any}
     function _RelationshipClass(name, intact_cls_names, object_tuples, vals=Dict(), defaults=Dict())
         cls_names = _fix_name_ambiguity(intact_cls_names)
-        row_map = Dict()
-        rc = new(name, intact_cls_names, cls_names, [], vals, defaults, row_map, _make_split_kwargs(cls_names))
+        rc = new(
+            name,
+            intact_cls_names,
+            cls_names,
+            [],
+            vals,
+            defaults,
+            Dict(),
+            ReentrantLock(),
+            _make_split_kwargs(cls_names),
+        )
         rels = [(; zip(cls_names, objects)...) for objects in object_tuples]
         _append_relationships!(rc, rels)
         rc
diff --git a/src/update_model.jl b/src/update_model.jl
index 2a70317..dbcdf3d 100644
--- a/src/update_model.jl
+++ b/src/update_model.jl
@@ -27,6 +27,8 @@ import .JuMP: MOI, MOIU, MutableArithmetics
 
 _Constant = Union{Number,UniformScaling}
 
+const _si_ext_lock = ReentrantLock()
+
 struct SpineInterfaceExt
     lower_bound::Dict{VariableRef,Any}
     upper_bound::Dict{VariableRef,Any}
@@ -34,6 +36,14 @@ struct SpineInterfaceExt
     SpineInterfaceExt() = new(Dict(), Dict(), Dict())
 end
 
+function _get_si_ext!(m)
+    lock(_si_ext_lock) do
+        get!(m.ext, :spineinterface) do
+            SpineInterfaceExt()
+        end
+    end
+end
+
 JuMP.copy_extension_data(data::SpineInterfaceExt, new_model::AbstractModel, model::AbstractModel) = nothing
 
 abstract type _CallSet <: MOI.AbstractScalarSet end
@@ -141,7 +151,7 @@ function _set_lower_bound(var, lb)
     if is_fixed(var)
         # Save bound
         m = owner_model(var)
-        ext = get!(m.ext, :spineinterface, SpineInterfaceExt())
+        ext = _get_si_ext!(m)
         ext.lower_bound[var] = lb
     elseif !isnan(lb)
         set_lower_bound(var, lb)
@@ -164,7 +174,7 @@ function _set_upper_bound(var, ub)
     if is_fixed(var)
         # Save bound
         m = owner_model(var)
-        ext = get!(m.ext, :spineinterface, SpineInterfaceExt())
+        ext = _get_si_ext!(m)
         ext.upper_bound[var] = ub
     elseif !isnan(ub)
         set_upper_bound(var, ub)
@@ -189,7 +199,7 @@ _fix(_upd, ::Nothing) = nothing
 function _fix(upd, fix_value)
     var = upd.variable
     m = owner_model(var)
-    ext = get!(m.ext, :spineinterface, SpineInterfaceExt())
+    ext = _get_si_ext!(m)
     if !isnan(fix_value)
         # Save bounds, remove them and then fix the value
         if has_lower_bound(var)
diff --git a/src/util.jl b/src/util.jl
index 71f6b9c..c48b138 100644
--- a/src/util.jl
+++ b/src/util.jl
@@ -225,7 +225,9 @@ function _refresh_metadata!(pval::ParameterValue)
 end
 
 function _add_update(t::TimeSlice, timeout, upd)
-    t.updates[upd] = timeout
+    lock(t.updates_lock) do
+        t.updates[upd] = timeout
+    end
 end
 
 function _append_relationships!(rc, rels)