We just solved this problem. In our case, the test database had unexpected data. (!!) It might have had a slightly different schema than expected.

The fix was to recreate the test database with MIX_ENV=test mix ecto.reset.

It was a frustrating experience.

I’m also experiencing this issue. and @dogweather’s solution didn’t help.
Can anyone please suggest how to fix this?
This is really weird.

My app is simple (non-umbrella) with the following relevant deps:
{:phoenix, “~> 1.5.1”},
{:phoenix_ecto, “~> 4.1”},
{:ecto_sql, “~> 3.4”},
{:postgrex, “>= 0.0.0”},

BTW, my OS :
Linux 4.15.0-1-amd64 #1 SMP Debian 4.15.4-1~mx17+1 (2018-02-23) x86_64 GNU/Linux

Distributor ID: MX
Description: MX 18.3 Continuum
Release: 18.3
Codename: Continuum

UPDATE: tried the following, but issue persists, preventing me from further development of my app:

mix deps.update --all   # Ok
mix ecto.reset    # Fail: this keeps throwing the same error (the current topic)

Also dropped both my_dev and my_test DBs via psql then ran mix ecto.reset to no success.

This comes up sometimes depending on how you are using your repo. Sometimes you just need to start it, e.g.

MyApp.Repo.start_link()