diff --git a/src/c/error.d b/src/c/error.d
index 8e2359864..7e4c1b62b 100644
--- a/src/c/error.d
+++ b/src/c/error.d
@@ -287,7 +287,7 @@ FEwrong_type_only_arg(cl_object function, cl_object value, cl_object type)
   struct ecl_ihs_frame tmp_ihs;
   function = cl_symbol_or_object(function);
   type = cl_symbol_or_object(type);
-  if (!Null(function) && env->ihs_top && env->ihs_top->function != function) {
+  if (!Null(function) && env->ihs_stack.top && env->ihs_stack.top->function != function) {
     ecl_ihs_push(env,&tmp_ihs,function,ECL_NIL);
   }        
   si_signal_simple_error(8,
@@ -311,7 +311,7 @@ FEwrong_type_nth_arg(cl_object function, cl_narg narg, cl_object value, cl_objec
   struct ecl_ihs_frame tmp_ihs;
   function = cl_symbol_or_object(function);
   type = cl_symbol_or_object(type);
-  if (!Null(function) && env->ihs_top && env->ihs_top->function != function) {
+  if (!Null(function) && env->ihs_stack.top && env->ihs_stack.top->function != function) {
     ecl_ihs_push(env,&tmp_ihs,function,ECL_NIL);
   }        
   si_signal_simple_error(8,
@@ -337,7 +337,7 @@ FEwrong_type_key_arg(cl_object function, cl_object key, cl_object value, cl_obje
   function = cl_symbol_or_object(function);
   type = cl_symbol_or_object(type);
   key = cl_symbol_or_object(key);
-  if (!Null(function) && env->ihs_top && env->ihs_top->function != function) {
+  if (!Null(function) && env->ihs_stack.top && env->ihs_stack.top->function != function) {
     ecl_ihs_push(env,&tmp_ihs,function,ECL_NIL);
   }        
   si_signal_simple_error(8,
@@ -368,7 +368,7 @@ FEwrong_index(cl_object function, cl_object a, int which, cl_object ndx,
   cl_env_ptr env = ecl_process_env();
   struct ecl_ihs_frame tmp_ihs;
   function = cl_symbol_or_object(function);
-  if (!Null(function) && env->ihs_top && env->ihs_top->function != function) {
+  if (!Null(function) && env->ihs_stack.top && env->ihs_stack.top->function != function) {
     ecl_ihs_push(env,&tmp_ihs,function,ECL_NIL);
   }        
   cl_error(9,
diff --git a/src/c/stacks.d b/src/c/stacks.d
index 23a9d0bc3..84ec64edf 100644
--- a/src/c/stacks.d
+++ b/src/c/stacks.d
@@ -585,7 +585,7 @@ static ecl_ihs_ptr
 get_ihs_ptr(cl_index n)
 {
   cl_env_ptr env = ecl_process_env();
-  ecl_ihs_ptr p = env->ihs_top;
+  ecl_ihs_ptr p = env->ihs_stack.top;
   if (n > p->index)
     FEerror("~D is an illegal IHS index.", 1, ecl_make_fixnum(n));
   while (n < p->index)
@@ -597,7 +597,7 @@ cl_object
 si_ihs_top(void)
 {
   cl_env_ptr env = ecl_process_env();
-  ecl_return1(env, ecl_make_fixnum(env->ihs_top->index));
+  ecl_return1(env, ecl_make_fixnum(env->ihs_stack.top->index));
 }
 
 cl_object
@@ -702,7 +702,7 @@ _ecl_frs_push(cl_env_ptr env)
   AO_nop_full();
   ++env->frs_stack.top;
   output->frs_bds_top_index = env->bds_stack.top - env->bds_stack.org;
-  output->frs_ihs = env->ihs_top;
+  output->frs_ihs = env->ihs_stack.top;
   output->frs_sp = ECL_STACK_INDEX(env);
   return output;
 }
@@ -716,7 +716,7 @@ ecl_unwind(cl_env_ptr env, ecl_frame_ptr fr)
     top->frs_val = ECL_DUMMY_TAG;
     --top;
   }
-  env->ihs_top = top->frs_ihs;
+  env->ihs_stack.top = top->frs_ihs;
   ecl_bds_unwind(env, top->frs_bds_top_index);
   ECL_STACK_SET_INDEX(env, top->frs_sp);
   env->frs_stack.top = top;
@@ -880,7 +880,7 @@ init_stacks(cl_env_ptr env)
   env->bds_stack.top = env->bds_stack.org-1;
   env->bds_stack.limit = &env->bds_stack.org[size - 2*margin];
   /* ihs stack */
-  env->ihs_top = &ihs_org;
+  env->ihs_stack.top = &ihs_org;
   ihs_org.function = ECL_NIL;
   ihs_org.lex_env = ECL_NIL;
   ihs_org.index = 0;
diff --git a/src/h/external.h b/src/h/external.h
index e903ad449..1d32394c5 100755
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -38,6 +38,12 @@ struct ecl_frames_stack {
         struct ecl_frame *limit;
 };
 
+/* The Invocation History Stack (IHS) keeps a list of the names of the functions
+ * that are invoked, together with their lexical environments. */
+struct ecl_history_stack {
+        struct ecl_ihs_frame *top;
+};
+
 /*
  * Per-thread data.
  */
@@ -70,15 +76,8 @@ struct cl_env_struct {
         cl_object *stack_limit;
 
         struct ecl_binding_stack bds_stack;
-        struct ecl_frames_stack  frs_stack;
-
-        /*
-         * The Invocation History Stack (IHS) keeps a list of the names of the
-         * functions that are invoked, together with their lexical
-         * environments.
-         */
-        struct ecl_ihs_frame *ihs_top;
-
+        struct ecl_frames_stack frs_stack;
+        struct ecl_history_stack ihs_stack;
 
         /*
          * The following pointers to the C Stack are used to ensure that a
diff --git a/src/h/stacks.h b/src/h/stacks.h
index 531516aca..a7698927c 100755
--- a/src/h/stacks.h
+++ b/src/h/stacks.h
@@ -240,18 +240,18 @@ typedef struct ecl_ihs_frame {
 #define ecl_ihs_push(env,rec,fun,lisp_env) do { \
         const cl_env_ptr __the_env = (env);     \
         ecl_ihs_ptr const r = (ecl_ihs_ptr const)(rec); \
-        r->next=__the_env->ihs_top;             \
-        r->function=(fun);                      \
-        r->lex_env=(lisp_env);                  \
-        r->index=__the_env->ihs_top->index+1;   \
+        r->next=__the_env->ihs_stack.top;             \
+        r->function=(fun);                            \
+        r->lex_env=(lisp_env);                        \
+        r->index=__the_env->ihs_stack.top->index+1;   \
         r->bds=__the_env->bds_stack.top - __the_env->bds_stack.org; \
-        __the_env->ihs_top = r;                 \
+        __the_env->ihs_stack.top = r;                               \
 } while(0)
 
 #define ecl_ihs_pop(env) do {                           \
         const cl_env_ptr __the_env = (env);             \
-        ecl_ihs_ptr r = __the_env->ihs_top;     \
-        if (r) __the_env->ihs_top = r->next;            \
+        ecl_ihs_ptr r = __the_env->ihs_stack.top;       \
+        if (r) __the_env->ihs_stack.top = r->next;      \
 } while(0)
 
 /***************