diff --git a/Makefile.am b/Makefile.am
index 99250803b7..e4c6d3c1c4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1825,16 +1825,17 @@ endif
 # sources only.
 sieve_libcyrus_sieve_lex_la_SOURCES = \
     sieve/addr-lex.l \
-    sieve/sieve-lex.l
+    sieve/sieve-lex.l \
+    sieve/addr.y \
+    sieve/sieve.y
 sieve_libcyrus_sieve_lex_la_LIBADD =
-sieve_libcyrus_sieve_lex_la_CFLAGS = $(AM_CFLAGS) $(CFLAG_VISIBILITY) $(NOWARN_SIGN_COMPARE)
+sieve_libcyrus_sieve_lex_la_CFLAGS = $(AM_CFLAGS) $(CFLAG_VISIBILITY) $(NOWARN_SIGN_COMPARE) $(NOWARN_UNUSED_BUT_SET)
 
 nodist_sieve_libcyrus_sieve_la_SOURCES = \
     sieve/sieve_err.c \
     sieve/sieve_err.h
 sieve_libcyrus_sieve_la_SOURCES = \
     sieve/bytecode.h \
-    sieve/addr.y \
     sieve/bc_emit.c \
     sieve/bc_eval.c \
     sieve/bc_generate.c \
@@ -1852,7 +1853,6 @@ sieve_libcyrus_sieve_la_SOURCES = \
     sieve/message.h \
     sieve/script.c \
     sieve/script.h \
-    sieve/sieve.y \
     sieve/tree.c \
     sieve/tree.h \
     sieve/variables.c \
diff --git a/configure.ac b/configure.ac
index dc6f998983..7ddf497a65 100644
--- a/configure.ac
+++ b/configure.ac
@@ -873,6 +873,21 @@ AC_COMPILE_IFELSE(
 CFLAGS="$saved_CFLAGS"
 AC_SUBST([NOWARN_SIGN_COMPARE])
 
+dnl look for an option to disable sign-comparison warnings (needed for
+dnl flex-generated sieve sources when building with -Werror)
+saved_CFLAGS="$CFLAGS"
+NOWARN_UNUSED_BUT_SET="-Wno-unused-but-set-variable"
+CFLAGS="$NOWARN_UNUSED_BUT_SET"
+AC_MSG_CHECKING([whether the C compiler supports -Wno-unused-but-set-variable])
+AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([], [])],
+    [AC_MSG_RESULT([yes])],
+    [NOWARN_UNUSED_BUT_SET=]
+    [AC_MSG_RESULT([no])]
+)
+CFLAGS="$saved_CFLAGS"
+AC_SUBST([NOWARN_UNUSED_BUT_SET])
+
 dnl for et routines
 AC_FUNC_CHECK(strerror,AC_DEFINE(HAS_STRERROR,[],[Do we have strerror()?]),
               AC_DEFINE(NEED_SYS_ERRLIST,[],[Do we have a sys_errlist?]))