diff --git a/bin/brew b/bin/brew
index 0fabbb97ef..f73ba14e14 100755
--- a/bin/brew
+++ b/bin/brew
@@ -7,10 +7,13 @@ quiet_cd() {
 
 BREW_FILE_DIRECTORY="$(quiet_cd "${0%/*}/" && pwd -P)"
 HOMEBREW_BREW_FILE="${BREW_FILE_DIRECTORY%/}/${0##*/}"
-
 HOMEBREW_PREFIX="${HOMEBREW_BREW_FILE%/*/*}"
-[[ -n "$HOMEBREW_PREFIX" && "$HOMEBREW_PREFIX" != "$HOMEBREW_BREW_FILE" ]] \
-  || HOMEBREW_PREFIX="/"
+
+# Default to / prefix if unset or the bin/brew file.
+if [[ -z "$HOMEBREW_PREFIX" || "$HOMEBREW_PREFIX" = "$HOMEBREW_BREW_FILE" ]]
+then
+  HOMEBREW_PREFIX="/"
+fi
 
 HOMEBREW_REPOSITORY="$HOMEBREW_PREFIX"