diff --git a/README.md b/README.md
index a0bfa78..7e3e674 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ If you invoke the `forward-word` widget, it will partially accept the suggestion
 
 ## Configuration
 
-You may want to override the default global config variables after sourcing the plugin. Default values of these variables can be found [here](src/config.zsh).
+You may want to override the default global config variables. Default values of these variables can be found [here](src/config.zsh).
 
 **Note:** If you are using Oh My Zsh, you can put this configuration in a file in the `$ZSH_CUSTOM` directory. See their comments on [overriding internals](https://github.com/robbyrussell/oh-my-zsh/wiki/Customization#overriding-internals).
 
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index bfcb706..abea917 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -11,8 +11,8 @@ RSpec.shared_context 'terminal session' do
 
   around do |example|
     before_sourcing.call
-
-    session.run_command((['source zsh-autosuggestions.zsh'] + options).join('; '))
+    session.run_command(options.join('; '))
+    session.run_command('source zsh-autosuggestions.zsh')
     after_sourcing.call
     session.clear_screen
 
diff --git a/src/config.zsh b/src/config.zsh
index b1e2c5b..9ac1484 100644
--- a/src/config.zsh
+++ b/src/config.zsh
@@ -6,17 +6,17 @@
 # Color to use when highlighting suggestion
 # Uses format of `region_highlight`
 # More info: http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#Zle-Widgets
-ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE='fg=8'
+: ${ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE='fg=8'}
 
 # Prefix to use when saving original versions of bound widgets
-ZSH_AUTOSUGGEST_ORIGINAL_WIDGET_PREFIX=autosuggest-orig-
+: ${ZSH_AUTOSUGGEST_ORIGINAL_WIDGET_PREFIX=autosuggest-orig-}
 
 # Strategies to use to fetch a suggestion
 # Will try each strategy in order until a suggestion is returned
-ZSH_AUTOSUGGEST_STRATEGY=(history)
+(( ! ${+ZSH_AUTOSUGGEST_STRATEGY} )) && ZSH_AUTOSUGGEST_STRATEGY=(history)
 
 # Widgets that clear the suggestion
-ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_CLEAR_WIDGETS} )) && ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
 	history-search-forward
 	history-search-backward
 	history-beginning-search-forward
@@ -31,7 +31,7 @@ ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
 )
 
 # Widgets that accept the entire suggestion
-ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_ACCEPT_WIDGETS} )) && ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
 	forward-char
 	end-of-line
 	vi-forward-char
@@ -40,11 +40,11 @@ ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
 )
 
 # Widgets that accept the entire suggestion and execute it
-ZSH_AUTOSUGGEST_EXECUTE_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_EXECUTE_WIDGETS} )) && ZSH_AUTOSUGGEST_EXECUTE_WIDGETS=(
 )
 
 # Widgets that accept the suggestion as far as the cursor moves
-ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS} )) && ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
 	forward-word
 	emacs-forward-word
 	vi-forward-word
@@ -56,7 +56,7 @@ ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
 )
 
 # Widgets that should be ignored (globbing supported but must be escaped)
-ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_IGNORE_WIDGETS} )) && ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
 	orig-\*
 	beep
 	run-help
@@ -66,8 +66,8 @@ ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
 	yank-pop
 )
 
-# Max size of buffer to trigger autosuggestion. Leave undefined for no upper bound.
-ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE=
+# Max size of buffer to trigger autosuggestion. Leave null for no upper bound.
+: ${ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE=}
 
 # Pty name for calculating autosuggestions asynchronously
-ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=zsh_autosuggest_pty
+: ${ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=zsh_autosuggest_pty}
diff --git a/zsh-autosuggestions.zsh b/zsh-autosuggestions.zsh
index 91f3af6..56aa80b 100644
--- a/zsh-autosuggestions.zsh
+++ b/zsh-autosuggestions.zsh
@@ -42,17 +42,17 @@ zmodload zsh/zpty
 # Color to use when highlighting suggestion
 # Uses format of `region_highlight`
 # More info: http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#Zle-Widgets
-ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE='fg=8'
+: ${ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE='fg=8'}
 
 # Prefix to use when saving original versions of bound widgets
-ZSH_AUTOSUGGEST_ORIGINAL_WIDGET_PREFIX=autosuggest-orig-
+: ${ZSH_AUTOSUGGEST_ORIGINAL_WIDGET_PREFIX=autosuggest-orig-}
 
 # Strategies to use to fetch a suggestion
 # Will try each strategy in order until a suggestion is returned
-ZSH_AUTOSUGGEST_STRATEGY=(history)
+(( ! ${+ZSH_AUTOSUGGEST_STRATEGY} )) && ZSH_AUTOSUGGEST_STRATEGY=(history)
 
 # Widgets that clear the suggestion
-ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_CLEAR_WIDGETS} )) && ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
 	history-search-forward
 	history-search-backward
 	history-beginning-search-forward
@@ -67,7 +67,7 @@ ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(
 )
 
 # Widgets that accept the entire suggestion
-ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_ACCEPT_WIDGETS} )) && ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
 	forward-char
 	end-of-line
 	vi-forward-char
@@ -76,11 +76,11 @@ ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(
 )
 
 # Widgets that accept the entire suggestion and execute it
-ZSH_AUTOSUGGEST_EXECUTE_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_EXECUTE_WIDGETS} )) && ZSH_AUTOSUGGEST_EXECUTE_WIDGETS=(
 )
 
 # Widgets that accept the suggestion as far as the cursor moves
-ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS} )) && ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
 	forward-word
 	emacs-forward-word
 	vi-forward-word
@@ -92,7 +92,7 @@ ZSH_AUTOSUGGEST_PARTIAL_ACCEPT_WIDGETS=(
 )
 
 # Widgets that should be ignored (globbing supported but must be escaped)
-ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
+(( ! ${+ZSH_AUTOSUGGEST_IGNORE_WIDGETS} )) && ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
 	orig-\*
 	beep
 	run-help
@@ -102,11 +102,11 @@ ZSH_AUTOSUGGEST_IGNORE_WIDGETS=(
 	yank-pop
 )
 
-# Max size of buffer to trigger autosuggestion. Leave undefined for no upper bound.
-ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE=
+# Max size of buffer to trigger autosuggestion. Leave null for no upper bound.
+: ${ZSH_AUTOSUGGEST_BUFFER_MAX_SIZE=}
 
 # Pty name for calculating autosuggestions asynchronously
-ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=zsh_autosuggest_pty
+: ${ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=zsh_autosuggest_pty}
 
 #--------------------------------------------------------------------#
 # Utility Functions                                                  #