diff --git a/bin/dotfiles b/bin/dotfiles
index 98e9825eca..58f27836cd 100755
--- a/bin/dotfiles
+++ b/bin/dotfiles
@@ -132,17 +132,24 @@ function _prompt_menu_add() {
 # (and not in a good way).
 # From http://stackoverflow.com/a/1617303/142339
 function setdiff() {
-  local debug oldifs A B C x
+  local debug skip a b
   if [[ "$1" == 1 ]]; then debug=1; shift; fi
-  # Convert args to arrays.
-  A=($1); B=($2); C=()
-  for x in "${A[@]}"; do
-    [[ "${B[*]}" =~ (^| )$x($| ) ]] || C=("${C[@]}" "$x")
+  if [[ "$1" ]]; then
+    local setdiffA setdiffB setdiffC
+    setdiffA=($1); setdiffB=($2)
+  fi
+  setdiffC=()
+  for a in "${setdiffA[@]}"; do
+    skip=
+    for b in "${setdiffB[@]}"; do
+      [[ "$a" == "$b" ]] && skip=1 && break
+    done
+    [[ "$skip" ]] || setdiffC=("${setdiffC[@]}" "$a")
   done
-  [[ "$debug" ]] && for x in A B C; do
-    echo "$x ($(eval echo "\${#$x[*]}")) $(eval echo "\${$x[*]}")" 1>&2
+  [[ "$debug" ]] && for a in setdiffA setdiffB setdiffC; do
+    echo "$a ($(eval echo "\${#$a[*]}")) $(eval echo "\${$a[*]}")" 1>&2
   done
-  echo "${C[@]}"
+  [[ "$1" ]] && echo "${setdiffC[@]}"
 }
 
 # If this file was being sourced, exit now.
diff --git a/test/test_setdiff.sh b/test/test_setdiff.sh
index 0cb5653164..e9569d46be 100755
--- a/test/test_setdiff.sh
+++ b/test/test_setdiff.sh
@@ -22,3 +22,27 @@ desired=(a a-b); installed=(a-b a); assert "" my_test
 desired=(a-b a); installed=(a-b a); assert "" my_test
 desired=(a a-b); installed=(a a-b); assert "" my_test
 desired=(a-b a); installed=(a a-b); assert "" my_test
+
+unset setdiffA setdiffB setdiffC
+setdiff "a b c" "" >/dev/null
+assert "0" echo "${#setdiffC[@]}"
+
+unset setdiffA setdiffB setdiffC; setdiffA=(a b c); setdiffB=(); setdiff
+assert "3" echo "${#setdiffC[@]}"
+assert "a b c" echo "${setdiffC[*]}"
+
+unset setdiffA setdiffB setdiffC; setdiffA=(a b c); setdiffB=(a); setdiff
+assert "2" echo "${#setdiffC[@]}"
+assert "b c" echo "${setdiffC[*]}"
+
+unset setdiffA setdiffB setdiffC; setdiffA=(a b c); setdiffB=(c a); setdiff
+assert "1" echo "${#setdiffC[@]}"
+assert "b" echo "${setdiffC[*]}"
+
+unset setdiffA setdiffB setdiffC; setdiffA=("a b" c); setdiffB=(a b c); setdiff
+assert "1" echo "${#setdiffC[@]}"
+assert "a b" echo "${setdiffC[*]}"
+
+unset setdiffA setdiffB setdiffC; setdiffA=(a b "a b" c "c d"); setdiffB=(a c); setdiff
+assert "3" echo "${#setdiffC[@]}"
+assert "b a b c d" echo "${setdiffC[*]}"