diff --git a/top/maint.mk b/top/maint.mk
index 7dfc157..868e10f 100644
--- a/top/maint.mk
+++ b/top/maint.mk
@@ -968,7 +968,7 @@ prohibit_doubled_words_ = \
 prohibit_doubled_word_expanded_ = \
     $(join $(prohibit_doubled_words_),$(addprefix \s+,$(prohibit_doubled_words_)))
 prohibit_doubled_word_RE_ ?= \
-    /\b(?:$(subst $(space),|,$(prohibit_doubled_word_expanded_)))\b/gims
+    /\b(?:$(subst $(_sp),|,$(prohibit_doubled_word_expanded_)))\b/gims
 prohibit_doubled_word_ =						\
     -e 'while ($(prohibit_doubled_word_RE_))'				\
     $(perl_filename_lineno_text_)