CrateDB output plugin fails with memory input plugin

[nomicode]

Bug report

Relevant telegraf.conf:

# Read metrics about memory usage
[[inputs.mem]]
  # no configuration

System info:

CrateDB 2.3

tested failing with two versions of Telegraf

installed via Homebrew on macOS 10.13.4:

$ telegraf --version
Telegraf v1.6.1 (git: unknown unknown)

installed via go get on macOS 10.13.4:

$ ./telegraf --version
Telegraf v1.7.0~d96bcac3 (git: master d96bcac3)

Steps to reproduce:

1. telegraf --input-filter mem --output-filter cratedb config > telegraf.conf
2. configure the CrateDB connection string
3. telegraf --config telegraf.conf

Expected behavior:

Telegraf runs with no errors, and the CrateDB table is created and populated

Actual behavior:

repeated messages like this:

2018-05-05T20:51:42Z E! Error writing to output [cratedb]: unexpected type: uint64: 0x16618f000

the table is created, but no records are written to CrateDB

Additional info:

this does not happen with the cpu input plugin

I have not tested any other plugins