diff --git a/cmake/z3_add_cxx_flag.cmake b/cmake/z3_add_cxx_flag.cmake
index 8bffd7de3..6e756d3b9 100644
--- a/cmake/z3_add_cxx_flag.cmake
+++ b/cmake/z3_add_cxx_flag.cmake
@@ -8,6 +8,7 @@ function(z3_add_cxx_flag flag)
   string(REPLACE "=" "_" SANITIZED_FLAG_NAME "${SANITIZED_FLAG_NAME}")
   string(REPLACE " " "_" SANITIZED_FLAG_NAME "${SANITIZED_FLAG_NAME}")
   string(REPLACE ":" "_" SANITIZED_FLAG_NAME "${SANITIZED_FLAG_NAME}")
+  string(REPLACE "+" "_" SANITIZED_FLAG_NAME "${SANITIZED_FLAG_NAME}")
   unset(HAS_${SANITIZED_FLAG_NAME})
   CHECK_CXX_COMPILER_FLAG("${flag}" HAS_${SANITIZED_FLAG_NAME})
   if (z3_add_flag_REQUIRED AND NOT HAS_${SANITIZED_FLAG_NAME})