diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index 74805d05..fe841e8e 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -370,15 +370,6 @@ typedef struct {
 #define alloc_bignum(data, p) \
   bignum_type *p = gc_alloc_bignum((gc_thread_data *)data);
 
-// TODO: the following macro is obsolete
-#define make_empty_bignum(n) \
-  bignum_type n; \
-  n.hdr.mark = gc_color_red; \
-  n.hdr.grayed = 0; \
-  n.tag = bignum_tag; \
-  mp_init(&(n.bn));
-/* TODO: check return value of mp_init */
-
 typedef struct {
   gc_header_type hdr;
   tag_type tag;
diff --git a/runtime.c b/runtime.c
index 440c015b..ea112f3b 100644
--- a/runtime.c
+++ b/runtime.c
@@ -1898,12 +1898,11 @@ object Cyc_string2number_(void *data, object cont, object str)
     if (rv == STR2INT_SUCCESS) {
       _return_closcall1(data, cont, obj_int2obj(result));
     } else if (str_is_bignum(rv, s)) {
-      make_empty_bignum(bn);
-      
-      // TODO: check return value
-      mp_read_radix(&(bignum_value(&bn)), s, 10);
-
-      _return_closcall1(data, cont, &bn);
+      alloc_bignum(data, bn);
+      if (MP_OKAY != mp_read_radix(&(bignum_value(bn)), s, 10)) {
+        Cyc_rt_raise2(data, "Error converting string to bignum", str);
+      }
+      _return_closcall1(data, cont, bn);
     } else {
       char *str_end;
       n = strtold(s, &str_end);
@@ -3153,10 +3152,10 @@ void Cyc_expt(void *data, object cont, object x, object y)
       if (obj_obj2int(y) < 0) {
         Cyc_expt_double(data, cont, (double)obj_obj2int(x), (double)obj_obj2int(y));
       } else {
-        make_empty_bignum(bn);
-        Cyc_int2bignum(obj_obj2int(x), &(bn.bn));
-        mp_expt_d(&bignum_value(&bn), obj_obj2int(y), &bignum_value(&bn));
-        return_closcall1(data, cont, Cyc_bignum_normalize(data, &bn));
+        alloc_bignum(data, bn);
+        Cyc_int2bignum(obj_obj2int(x), &(bn->bn));
+        mp_expt_d(&bignum_value(bn), obj_obj2int(y), &bignum_value(bn));
+        return_closcall1(data, cont, Cyc_bignum_normalize(data, bn));
       }
     } else if (is_object_type(y) && type_of(y) == double_tag) {
       Cyc_expt_double(data, cont, (double)obj_obj2int(x), double_value(y));
@@ -3181,9 +3180,9 @@ void Cyc_expt(void *data, object cont, object x, object y)
       if (obj_obj2int(y) < 0) {
         Cyc_expt_double(data, cont, mp_get_double(&bignum_value(x)), (double)obj_obj2int(y));
       } else {
-        make_empty_bignum(bn);
-        mp_expt_d(&bignum_value(x), obj_obj2int(y), &bignum_value(&bn));
-        return_closcall1(data, cont, Cyc_bignum_normalize(data, &bn));
+        alloc_bignum(data, bn);
+        mp_expt_d(&bignum_value(x), obj_obj2int(y), &bignum_value(bn));
+        return_closcall1(data, cont, Cyc_bignum_normalize(data, bn));
       }
     } else if (is_object_type(y) && type_of(y) == double_tag) {
       Cyc_expt_double(data, cont, mp_get_double(&bignum_value(x)), double_value(y));