diff --git a/asteria/compiler/expression_unit.cpp b/asteria/compiler/expression_unit.cpp
index 74815dd5b..8443956b8 100644
--- a/asteria/compiler/expression_unit.cpp
+++ b/asteria/compiler/expression_unit.cpp
@@ -56,7 +56,7 @@ generate_code(cow_vector<AIR_Node>& code, const Compiler_Options& opts,
         // If a named reference is found, it will not be replaced or hidden by a
         // later-declared one.
         const Reference* qref;
-        Abstract_Context* qctx = &ctx;
+        const Abstract_Context* qctx = &ctx;
         uint32_t depth = 0;
 
         for(;;) {
diff --git a/asteria/runtime/abstract_context.hpp b/asteria/runtime/abstract_context.hpp
index 365a6ff17..cd2ed8089 100644
--- a/asteria/runtime/abstract_context.hpp
+++ b/asteria/runtime/abstract_context.hpp
@@ -32,7 +32,7 @@ class Abstract_Context
     do_is_analytic() const noexcept = 0;
 
     virtual
-    Abstract_Context*
+    const Abstract_Context*
     do_get_parent_opt() const noexcept = 0;
 
     // This function is called when a name is not found in `m_named_refs`.
@@ -57,7 +57,7 @@ class Abstract_Context
     is_analytic() const noexcept
       { return this->do_is_analytic();  }
 
-    Abstract_Context*
+    const Abstract_Context*
     get_parent_opt() const noexcept
       { return this->do_get_parent_opt();  }
 
diff --git a/asteria/runtime/air_node.cpp b/asteria/runtime/air_node.cpp
index 75f89a029..2ad1871fe 100644
--- a/asteria/runtime/air_node.cpp
+++ b/asteria/runtime/air_node.cpp
@@ -1009,7 +1009,7 @@ struct Traits_push_local_reference
     execute(Executive_Context& ctx, AVMC_Queue::Uparam up, phsh_stringR name)
       {
         // Get the context.
-        Executive_Context* qctx = &ctx;
+        const Executive_Context* qctx = &ctx;
         for(uint32_t k = 0;  k != up.u32;  ++k)
           qctx = qctx->get_parent_opt();
 
@@ -4888,7 +4888,7 @@ rebind_opt(Abstract_Context& ctx) const
         const auto& altr = this->m_stor.as<index_push_local_reference>();
 
         // Get the context.
-        Abstract_Context* qctx = &ctx;
+        const Abstract_Context* qctx = &ctx;
         for(uint32_t k = 0;  k != altr.depth;  ++k)
           qctx = qctx->get_parent_opt();
 
diff --git a/asteria/runtime/analytic_context.hpp b/asteria/runtime/analytic_context.hpp
index a48586f58..9235bbf70 100644
--- a/asteria/runtime/analytic_context.hpp
+++ b/asteria/runtime/analytic_context.hpp
@@ -13,7 +13,7 @@ class Analytic_Context
     public Abstract_Context
   {
   private:
-    Abstract_Context* m_parent_opt;
+    const Abstract_Context* m_parent_opt;
 
   public:
     // A plain context must have a parent context.
@@ -37,7 +37,7 @@ class Analytic_Context
     do_is_analytic() const noexcept final
       { return true;  }
 
-    Abstract_Context*
+    const Abstract_Context*
     do_get_parent_opt() const noexcept override
       { return this->m_parent_opt;  }
 
@@ -48,7 +48,7 @@ class Analytic_Context
   public:
     ASTERIA_NONCOPYABLE_DESTRUCTOR(Analytic_Context);
 
-    Abstract_Context*
+    const Abstract_Context*
     get_parent_opt() const noexcept
       { return this->m_parent_opt;  }
   };
diff --git a/asteria/runtime/executive_context.hpp b/asteria/runtime/executive_context.hpp
index f2a4c4a55..151705085 100644
--- a/asteria/runtime/executive_context.hpp
+++ b/asteria/runtime/executive_context.hpp
@@ -14,7 +14,7 @@ class Executive_Context
     public Abstract_Context
   {
   private:
-    Executive_Context* m_parent_opt;
+    const Executive_Context* m_parent_opt;
 
     // Store some references to the enclosing function,
     // so they are not passed here and there upon each native call.
@@ -64,7 +64,7 @@ class Executive_Context
     do_is_analytic() const noexcept final
       { return false;  }
 
-    Abstract_Context*
+    const Abstract_Context*
     do_get_parent_opt() const noexcept override
       { return this->m_parent_opt;  }
 
@@ -80,7 +80,7 @@ class Executive_Context
   public:
     ASTERIA_NONCOPYABLE_DESTRUCTOR(Executive_Context);
 
-    Executive_Context*
+    const Executive_Context*
     get_parent_opt() const noexcept
       { return this->m_parent_opt;  }
 
diff --git a/asteria/runtime/global_context.hpp b/asteria/runtime/global_context.hpp
index 8c2678b7c..942cb273d 100644
--- a/asteria/runtime/global_context.hpp
+++ b/asteria/runtime/global_context.hpp
@@ -32,7 +32,7 @@ class Global_Context
     do_is_analytic() const noexcept final
       { return false;  }
 
-    Abstract_Context*
+    const Abstract_Context*
     do_get_parent_opt() const noexcept final
       { return nullptr;  }