diff --git a/configure.ac b/configure.ac
index f7620268..321ab2c0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -197,7 +197,7 @@ dnl skipped and all flags rechecked.  So there's no need to do anything
 dnl else.  If for any reason you need to force a recheck, just change
 dnl MAYBE_WARN in an ignorable way (like adding whitespace)
 
-MAYBE_WARN="-Wall -Wextra -Wundef \
+MAYBE_WARN="-Wall -Wextra -Werror=undef \
 -Wsign-compare -Werror-implicit-function-declaration \
 -Wpointer-arith -Wwrite-strings -Wstrict-prototypes \
 -Wmissing-prototypes -Wmissing-declarations -Wnested-externs \
diff --git a/meson.build b/meson.build
index e249ea0f..1342a5b3 100644
--- a/meson.build
+++ b/meson.build
@@ -203,7 +203,7 @@ if cc.has_function('open_memstream')
 endif
 
 warn_c_args = []
-foreach a : ['-Wall', '-Wextra', '-Wsign-compare', '-Wundef',
+foreach a : ['-Wall', '-Wextra', '-Wsign-compare', '-Werror=undef',
              '-Werror-implicit-function-declaration', '-Wpointer-arith',
              '-Wwrite-strings', '-Wstrict-prototypes', '-Wmissing-prototypes',
              '-Wmissing-declarations', '-Wnested-externs', '-Wpacked',