diff --git a/R/util.R b/R/util.R
index 27f7d57..dc71524 100644
--- a/R/util.R
+++ b/R/util.R
@@ -61,6 +61,7 @@
   x <- gsub("\\}", "\\\\}", x)
   x <- gsub("\\*", "\\\\*", x)
   x <- gsub("\\+", "\\\\+", x)
+  x <- gsub("\\?", "\\\\?", x)
   return(x)
 }