diff --git a/configure.ac b/configure.ac
index 4010e7b264..d22bba15e2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -155,7 +155,7 @@ test -z "$optflags" ||
     cflagspat="$cflagspat;s|"`eval echo '"'"${optflags}"'"' | sed 's/[[][|.*]]/\\&/g;s/^ */ /;s/ *$/ /'`'| |g'
 test -z "$debugflags" ||
     cflagspat="$cflagspat;s|"`eval echo '"'"${debugflags}"'"' | sed 's/[[][|.*]]/\\&/g;s/^ */ /;s/ *$/ /'`'| |g'
-test -z "warnflags" ||
+test -z "$warnflags" ||
     cflagspat="$cflagspat;s|"`eval echo '"'"${warnflags}"'"' | sed 's/[[][|.*]]/\\&/g;s/^ */ /;s/ *$/ /'`'| |g'
 AS_IF([test -z "${CFLAGS+set}"], [
     cflags=`echo " $cflags " | sed "$cflagspat;s/^ *//;s/ *$//"`