diff --git a/zsh-history-substring-search.zsh b/zsh-history-substring-search.zsh
index 986b5f8..34c3df5 100644
--- a/zsh-history-substring-search.zsh
+++ b/zsh-history-substring-search.zsh
@@ -38,6 +38,25 @@
 #
 ##############################################################################
 
+#-----------------------------------------------------------------------------
+# declare global variables
+#-----------------------------------------------------------------------------
+
+typeset -g BUFFER MATCH MBEGIN MEND CURSOR
+typeset -g HISTORY_SUBSTRING_SEARCH_HIGHLIGHT_FOUND
+typeset -g HISTORY_SUBSTRING_SEARCH_HIGHLIGHT_NOT_FOUND
+typeset -g HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS
+typeset -g HISTORY_SUBSTRING_SEARCH_ENSURE_UNIQUE
+typeset -g _history_substring_search_refresh_display
+typeset -g _history_substring_search_query_highlight
+typeset -g _history_substring_search_result
+typeset -g _history_substring_search_query
+typeset -g -A _history_substring_search_raw_matches
+typeset -g _history_substring_search_raw_match_index
+typeset -g -A _history_substring_search_matches
+typeset -g -A _history_substring_search_unique_filter
+typeset -g _history_substring_search_match_index
+
 #-----------------------------------------------------------------------------
 # configuration variables
 #-----------------------------------------------------------------------------