diff --git a/autoload/gitgutter/diff_highlight.vim b/autoload/gitgutter/diff_highlight.vim
index 536e52e..803dd6e 100644
--- a/autoload/gitgutter/diff_highlight.vim
+++ b/autoload/gitgutter/diff_highlight.vim
@@ -162,7 +162,7 @@ function! s:common_prefix(a, b)
     return -1
   endif
   for i in range(len)
-    if a:a[i:i] != a:b[i:i]
+    if a:a[i:i] !=# a:b[i:i]
       return i - 1
     endif
   endfor