diff --git a/.travis.yml b/.travis.yml
index f6923abd..f7c4ba4f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,8 +41,8 @@ matrix:
         - PYVIPS_VERSION=master
         - JPEG=/usr
         - JOBS=`nproc`
-        - CFLAGS="-fsanitize=address -fno-omit-frame-pointer -fopenmp -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
-        - LDFLAGS="-fsanitize=address -dynamic-asan -fopenmp=libiomp5"
+        - CFLAGS="-fsanitize=address,undefined -fno-omit-frame-pointer -fopenmp -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
+        - LDFLAGS="-fsanitize=address,undefined -dynamic-asan -fopenmp=libiomp5"
         - ASAN_DSO=/usr/local/clang-7.0.0/lib/clang/7.0.0/lib/linux/libclang_rt.asan-x86_64.so
         - LSAN_OPTIONS="suppressions=$TRAVIS_BUILD_DIR/lsan.supp"
       install:
diff --git a/fuzz/test_fuzz.sh b/fuzz/test_fuzz.sh
index 02af3c48..dd4decb5 100755
--- a/fuzz/test_fuzz.sh
+++ b/fuzz/test_fuzz.sh
@@ -6,6 +6,7 @@ set -e
 # Glib is build without -fno-omit-frame-pointer. We need
 # to disable the fast unwinder to get full stacktraces.
 export ASAN_OPTIONS="fast_unwind_on_malloc=0:allocator_may_return_null=1"
+export UBSAN_OPTIONS="print_stacktrace=1"
 
 # Hide all warning messages from vips.
 export VIPS_WARNING=0