diff --git a/.travis.yml b/.travis.yml
index e3408cf6f..d728758ed 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -98,7 +98,7 @@ matrix:
               - p7zip-full
               - gcc-6
           compiler: gcc-6
-          env: WFLAGS="-Wno-error=tautological-compare"
+          env: WFLAGS="-Wno-tautological-compare"
           #gcc-6 (Ubuntu 6.1.1-3ubuntu11~14.04.1) 6.1.1 20160511
         - os: linux
           compiler: clang
@@ -213,7 +213,7 @@ before_script:
   - 7z x $HOME/srb2_cache/SRB2-v2115-assets-2.7z -oassets
   - mkdir build
   - cd build
-  - export CFLAGS="-Wall -W $WFLAGS"
+  - export CFLAGS="-Wall -W -Werror $WFLAGS"
   - export CCACHE_COMPRESS=true
   - cmake .. -DCMAKE_BUILD_TYPE=Release
 
diff --git a/src/p_setup.c b/src/p_setup.c
index 6c14e3614..e56c44c70 100644
--- a/src/p_setup.c
+++ b/src/p_setup.c
@@ -1934,7 +1934,7 @@ static void P_GroupLines(void)
 		if (sector->linecount == 0) // no lines found?
 		{
 			sector->lines = NULL;
-			CONS_Debug(DBG_SETUP, "P_GroupLines: sector %d has no lines\n", i);
+			CONS_Debug(DBG_SETUP, "P_GroupLines: sector %s has no lines\n", sizeu1(i));
 		}
 		else
 		{