diff --git a/src/aws_rds_iam_token.erl b/src/aws_rds_iam_token.erl
index d1a0b171..07df30c9 100644
--- a/src/aws_rds_iam_token.erl
+++ b/src/aws_rds_iam_token.erl
@@ -7,7 +7,11 @@
 %%====================================================================
 %% API
 %%====================================================================
-create(Client, DBEndpoint, DBPort, DBUser) ->
+-spec create(aws_client:client(), binary(), non_neg_integer(), binary()) -> {ok, binary()}.
+create(Client, DBEndpoint, DBPort, DBUser) when is_map(Client) andalso
+                                                is_binary(DBEndpoint) andalso
+                                                is_integer(DBPort) andalso
+                                                is_binary(DBUser) ->
   Method = <<"GET">>,
   QueryParams = [{<<"Action">>, <<"connect">>}, {<<"DBUser">>, DBUser}],
   Endpoint = <<"https://", DBEndpoint/binary, ":", (integer_to_binary(DBPort))/binary>>,