diff --git a/modules/history-substring-search/init.zsh b/modules/history-substring-search/init.zsh
index 53eb99b..0fc191d 100644
--- a/modules/history-substring-search/init.zsh
+++ b/modules/history-substring-search/init.zsh
@@ -10,7 +10,7 @@
 pmodload 'editor'
 
 # Source module files.
-source "${0:h}/external/zsh-history-substring-search.zsh"
+source "${0:h}/external/zsh-history-substring-search.zsh" || return 1
 
 #
 # Search
diff --git a/modules/syntax-highlighting/init.zsh b/modules/syntax-highlighting/init.zsh
index e5c555b..55172b8 100644
--- a/modules/syntax-highlighting/init.zsh
+++ b/modules/syntax-highlighting/init.zsh
@@ -11,7 +11,7 @@ if ! zstyle -t ':prezto:module:syntax-highlighting' color; then
 fi
 
 # Source module files.
-source "${0:h}/external/zsh-syntax-highlighting.zsh"
+source "${0:h}/external/zsh-syntax-highlighting.zsh" || return 1
 
 # Set highlighters.
 zstyle -a ':prezto:module:syntax-highlighting' highlighters 'ZSH_HIGHLIGHT_HIGHLIGHTERS'