Missing prebuilt package zlib/1.2.11@conan/stable

[jngd]

I'm trying to build an Android application using conan with the following conanfile.txt:

[requires]
Poco/1.10.1@pocoproject/stable

[options]
Poco:shared=True # False

[generators]
txt
cmake

In my build.gradle I have a task with conan install command:

def cmd = conan_path + "conan install ../conanfile.txt --profile android_21_arm_clang"

My android_21_arm_clang profile is:

target_host=arm-linux-androideabi
cc_compiler=clang
cxx_compiler=clang++

[settings]
compiler=clang
compiler.version=5.0
compiler.libcxx=libc++
os=Android
os.api_level=21
arch=armv7
build_type=Release

[env]
CONAN_CMAKE_FIND_ROOT_PATH=$standalone_toolchain/sysroot
PATH=[$standalone_toolchain/bin]
CHOST=$target_host
AR=$target_host-ar
AS=$target_host-as
RANLIB=$target_host-ranlib
CC=$target_host-$cc_compiler
CXX=$target_host-$cxx_compiler
LD=$target_host-ld
STRIP=$target_host-strip
CFLAGS= -fPIE -fPIC -I$standalone_toolchain/include/c++/4.9.x
CXXFLAGS= -fPIE -fPIC -I$standalone_toolchain/include/c++/4.9.x
LDFLAGS= -pie

I'm getting this error:

Missing prebuilt package for 'zlib/1.2.11@conan/stable'

Do I need to do something else to get this up and running?