diff --git a/.travis.yml b/.travis.yml
index a998f32..d3b3e4b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -298,7 +298,7 @@ matrix:
               key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
 
     - os: linux
-      dist: bionic
+      dist: xenial
       compiler: clang-9
       env: TOOLSET=clang COMPILER=clang++-9 CXXSTD64=03,11,14,17 CXXSTD32=03,11
       addons:
@@ -309,13 +309,14 @@ matrix:
             - linux-libc-dev:i386
           sources:
             - sourceline: "ppa:ubuntu-toolchain-r/test"
-            - sourceline: "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main"
+            - sourceline: "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-9 main"
               key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
 
+# Note: clang for 32-bit targets with UBSAN emits undefined references to __mulodi4
     - os: linux
-      dist: bionic
+      dist: xenial
       compiler: clang-UBSAN
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-9 CXXSTD64=03,11,14,17 CXXSTD32=03,11 UBSAN_OPTIONS=print_stacktrace=1
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-9 CXXSTD64=03,11,14,17 UBSAN_OPTIONS=print_stacktrace=1
       addons:
         apt:
           packages:
@@ -324,7 +325,7 @@ matrix:
             - linux-libc-dev:i386
           sources:
             - sourceline: "ppa:ubuntu-toolchain-r/test"
-            - sourceline: "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main"
+            - sourceline: "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-9 main"
               key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
 
 # clang, OS X, 64-bit
@@ -376,8 +377,8 @@ script:
   - |-
     echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
   - BUILD_JOBS=`(nproc || sysctl -n hw.ncpu) 2> /dev/null`
-  - if [ -n "$CXXSTD64" ]; then echo ""; echo "Testing 64-bit targets"; echo ""; ./b2 -j $BUILD_JOBS libs/atomic/test toolset=$TOOLSET address-model=64 cxxstd=$CXXSTD64 ${UBSAN:+cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined define=UBSAN=1 debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}; fi
-  - if [ -n "$CXXSTD32" ]; then echo ""; echo "Testing 32-bit targets"; echo ""; ./b2 -j $BUILD_JOBS libs/atomic/test toolset=$TOOLSET address-model=32 cxxstd=$CXXSTD32 ${UBSAN:+cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined define=UBSAN=1 debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}; fi
+  - if [ -n "$CXXSTD64" ]; then echo ""; echo "Testing 64-bit targets"; echo ""; ./b2 -j $BUILD_JOBS libs/atomic/test toolset=$TOOLSET address-model=64 cxxstd=$CXXSTD64 ${UBSAN:+cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined define=UBSAN=1 debug-symbols=on} ${CXXFLAGS:+cxxflags="$CXXFLAGS"} ${LINKFLAGS:+linkflags="$LINKFLAGS"}; fi
+  - if [ -n "$CXXSTD32" ]; then echo ""; echo "Testing 32-bit targets"; echo ""; ./b2 -j $BUILD_JOBS libs/atomic/test toolset=$TOOLSET address-model=32 cxxstd=$CXXSTD32 ${UBSAN:+cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined define=UBSAN=1 debug-symbols=on} ${CXXFLAGS:+cxxflags="$CXXFLAGS"} ${LINKFLAGS:+linkflags="$LINKFLAGS"}; fi
 
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