diff --git a/.travis.yml b/.travis.yml
index 4b51120af4..3f46053eec 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -46,7 +46,6 @@ matrix:
       compiler: gcc
       env:
         - GCC_VERSION=7
-        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=MinSizeRel"
       addons:
         apt:
           sources:
@@ -58,15 +57,15 @@ matrix:
     - os: linux
       compiler: clang
       env:
-        - CLANG_VERSION=3.9
-        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=Debug -DDYN_OPENAL=NO -DDYN_SNDFILE=NO -DDYN_MPG123=NO -DDYN_FLUIDSYNTH=NO"
+        - CLANG_VERSION=4.0
+        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=MinSizeRel -DDYN_OPENAL=NO -DDYN_SNDFILE=NO -DDYN_MPG123=NO -DDYN_FLUIDSYNTH=NO"
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-3.9
+            - llvm-toolchain-trusty-4.0
           packages:
-            - clang-3.9
+            - clang-4.0
             - libstdc++-5-dev
             - libsdl2-dev
             - libgme-dev
diff --git a/src/p_ceiling.cpp b/src/p_ceiling.cpp
index fcd2a54876..fb324b54a3 100644
--- a/src/p_ceiling.cpp
+++ b/src/p_ceiling.cpp
@@ -593,8 +593,9 @@ bool EV_CeilingCrushStop (int tag, bool remove)
 
 bool EV_StopCeiling(int tag, line_t *line)
 {
+	int sec;
 	FSectorTagIterator it(tag, line);
-	while (int sec = it.Next())
+	while ((sec = it.Next()) >= 0)
 	{
 		if (level.sectors[sec].ceilingdata)
 		{
diff --git a/src/p_floor.cpp b/src/p_floor.cpp
index 63ebdc4af6..d0b6cb0773 100644
--- a/src/p_floor.cpp
+++ b/src/p_floor.cpp
@@ -570,8 +570,9 @@ bool EV_FloorCrushStop (int tag, line_t *line)
 // same as above but stops any floor mover that was active on the given sector.
 bool EV_StopFloor(int tag, line_t *line)
 {
+	int sec;
 	FSectorTagIterator it(tag, line);
-	while (int sec = it.Next())
+	while ((sec = it.Next()) >= 0)
 	{
 		if (level.sectors[sec].floordata)
 		{