diff --git a/configure.ac b/configure.ac
index a37d7e16..06cf6532 100644
--- a/configure.ac
+++ b/configure.ac
@@ -669,6 +669,7 @@ if test "x$werror" != "xno"; then
     AX_CHECK_COMPILE_FLAG([-Wunreachable-code-break], [CFLAGS="$CFLAGS -Wunreachable-code-break"])
     AX_CHECK_COMPILE_FLAG([-Wunused-macros], [CFLAGS="$CFLAGS -Wunused-macros"])
     AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [CFLAGS="$CFLAGS -Wunused-parameter"])
+    AX_CHECK_COMPILE_FLAG([-Wredundant-decls], [CFLAGS="$CFLAGS -Wredundant-decls"])
     # Only work with Clang for the moment
     AX_CHECK_COMPILE_FLAG([-Wheader-guard], [CFLAGS="$CFLAGS -Wheader-guard"])
 fi
diff --git a/lib/nghttp2_hd_huffman.c b/lib/nghttp2_hd_huffman.c
index 591636bb..48638b75 100644
--- a/lib/nghttp2_hd_huffman.c
+++ b/lib/nghttp2_hd_huffman.c
@@ -30,9 +30,6 @@
 
 #include "nghttp2_hd.h"
 
-extern const nghttp2_huff_sym huff_sym_table[];
-extern const nghttp2_huff_decode huff_decode_table[][16];
-
 /*
  * Encodes huffman code |sym| into |*dest_ptr|, whose least |rembits|
  * bits are not filled yet.  The |rembits| must be in range [1, 8],
diff --git a/tests/nghttp2_session_test.c b/tests/nghttp2_session_test.c
index 70d1c0d8..c2376b30 100644
--- a/tests/nghttp2_session_test.c
+++ b/tests/nghttp2_session_test.c
@@ -36,8 +36,6 @@
 #include "nghttp2_test_helper.h"
 #include "nghttp2_priority_spec.h"
 
-extern int nghttp2_enable_strict_preface;
-
 #define OB_CTRL(ITEM) nghttp2_outbound_item_get_ctrl_frame(ITEM)
 #define OB_CTRL_TYPE(ITEM) nghttp2_outbound_item_get_ctrl_frame_type(ITEM)
 #define OB_DATA(ITEM) nghttp2_outbound_item_get_data_frame(ITEM)