diff --git a/beacon_chain/rpc/rest_utils.nim b/beacon_chain/rpc/rest_utils.nim
index e14713259..fc135729d 100644
--- a/beacon_chain/rpc/rest_utils.nim
+++ b/beacon_chain/rpc/rest_utils.nim
@@ -606,7 +606,9 @@ proc toValidatorIndex*(value: RestValidatorIndex): Result[ValidatorIndex,
                                                           ValidatorIndexError] =
   when sizeof(ValidatorIndex) == 4:
     if uint64(value) < VALIDATOR_REGISTRY_LIMIT:
-      if uint64(value) <= uint64(high(uint32)):
+      # On x86 platform Nim allows only `int32` indexes, so all the indexes in
+      # range `2^31 <= x < 2^32` are not supported.
+      if uint64(value) <= uint64(high(int32)):
         ok(ValidatorIndex(value))
       else:
         err(ValidatorIndexError.UnsupportedValue)