diff --git a/mrbgems/mruby-array-ext/src/array.c b/mrbgems/mruby-array-ext/src/array.c
index 41ed3612d..520327be1 100644
--- a/mrbgems/mruby-array-ext/src/array.c
+++ b/mrbgems/mruby-array-ext/src/array.c
@@ -1309,6 +1309,11 @@ ary_combination_init(mrb_state *mrb, mrb_value self)
   mrb_bool permutation;
 
   mrb_get_args(mrb, "ib", &n, &permutation);
+#if MRB_INT_MAX > SIZE_MAX
+  if (n > SIZE_MAX) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "number too large");
+  }
+#endif
 
   struct RData *d;
   struct mrb_combination_state *state;
@@ -1321,10 +1326,7 @@ ary_combination_init(mrb_state *mrb, mrb_value self)
   state->finished = (n <= 0 && n != 0);
 
   if (n > 0) {
-    state->indices = (mrb_int*)mrb_malloc(mrb, sizeof(mrb_int) * n);
-    for (mrb_int i = 0; i < n; i++) {
-      state->indices[i] = 0;
-    }
+    state->indices = (mrb_int*)mrb_calloc(mrb, n, sizeof(mrb_int));
   }
 
   return mrb_obj_value(d);
diff --git a/src/gc.c b/src/gc.c
index 825d54600..ef91432af 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -243,15 +243,17 @@ mrb_calloc(mrb_state *mrb, size_t nelem, size_t len)
 {
   void *p;
 
-  if (nelem > 0 && len > 0 &&
-      nelem <= SIZE_MAX / len) {
+  if (nelem == 0 || len == 0) {
+    p = NULL;
+  }
+  else if (nelem <= SIZE_MAX / len) {
     size_t size = nelem * len;
     p = mrb_malloc(mrb, size);
 
     memset(p, 0, size);
   }
   else {
-    p = NULL;
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "memory allocation overflow");
   }
 
   return p;