[Rails] gem install fails so no installation of Rails

On Ubuntu 12.04 trying to install Rails with
$ gem install Rails
fails with
ERROR:  Loading command: install (LoadError)
    cannot load such file -- zlib
ERROR:  While executing gem ... (NameError)
    uninitialized constant Gem::Commands::InstallCommand

Ruby was installed with rbenv. More info:

~$ gem environment
RubyGems Environment:
  - RUBYGEMS VERSION: 1.8.23
  - RUBY VERSION: 1.9.3 (2012-10-12 patchlevel 286) [i686-linux]
  - INSTALLATION DIRECTORY: /home/user/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1
  - RUBY EXECUTABLE: /home/user/.rbenv/versions/1.9.3-p286/bin/ruby
  - EXECUTABLE DIRECTORY: /home/user/.rbenv/versions/1.9.3-p286/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-linux
  - GEM PATHS:
     - /home/user/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1
     - /home/user/.gem/ruby/1.9.1
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/

and

~$ lsb_release -a;uname -a; dpkg -l|grep zlib
LSB Version:    core-2.0-ia32:core-2.0-noarch:core-3.0-ia32:core-3.0-noarch:core-3.1-ia32:core-3.1-noarch:core-3.2-ia32:core-3.2-noarch:core-4.0-ia32:core-4.0-noarch
Distributor ID:    Ubuntu
Description:    Ubuntu 12.04.1 LTS
Release:    12.04
Codename:    precise
Linux machine 3.2.0-32-generic-pae #51-Ubuntu SMP Wed Sep 26 21:54:23 UTC 2012 i686 i686 i386 GNU/Linux
ii  zlib1g                                 1:1.2.3.4.dfsg-3ubuntu4                 compression library - runtime
ii  zlib1g-dev                             1:1.2.3.4.dfsg-3ubuntu4                 compression library - development

What is wrong?

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