Reactors should be in an array

salt/files/master.d/reactor.conf

 # This file is managed by Salt! Do not edit by hand!
 #
 {# The parameter reactor is kept for backward compatibility -#}
-{%- set reactors = salt['pillar.get']('salt:reactor') -%}
+{%- set reactors = salt['pillar.get']('salt:reactor', []) -%}
 {%- set reactors = salt['pillar.get']('salt:reactors', default=reactors, merge=True) -%}
 {%- set reactors = salt['pillar.get']('salt:master:reactors', default=reactors, merge=True) -%}
 

salt/files/minion.d/reactor.conf

 # This file is managed by Salt! Do not edit by hand!
 #
 {# The parameter reactor is kept for backward compatibility -#}
-{%- set reactors = salt['pillar.get']('salt:reactor') -%}
+{%- set reactors = salt['pillar.get']('salt:reactor', []) -%}
 {%- set reactors = salt['pillar.get']('salt:reactors', default=reactors, merge=True) -%}
 {%- set reactors = salt['pillar.get']('salt:minion:reactors', default=reactors, merge=True) -%}