Imagemagick 7 policy

ImageMagick-7 Changed the default policy to not allow writing to STDOUT

to modify the policy you can simply run the following command with sudo priviledges.

sudo sed -i 's/\(<policy domain="path" rights="\)none\(" pattern="-"\/>\)/\1write\2/' /etc/ImageMagick-7/policy.xml

Detail:
on the file /etc/ImageMagick-7/policy.xml find the line with <policy domain="path" rights="none" pattern="-"/> and change rights="none" to rights="write"

thanks to @Dreded for finding the root of the issue and submitting the solution in pywal: #729