diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0a6a9accea..beca0c1d5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -253,6 +253,10 @@ option(
 if(TRAVIS_CI_BUILD)
   message(STATUS "Travis CI build enabled.")
   add_definitions(-Werror)
+  if(DEFINED ENV{BUILD_32BIT})
+    # Get some test coverage for unsigned char
+    add_definitions(-funsigned-char)
+  endif()
 endif()
 
 if(CMAKE_BUILD_TYPE MATCHES Debug)