diff --git a/repos/spack_repo/builtin/packages/boost/package.py b/repos/spack_repo/builtin/packages/boost/package.py
index d8b0442b034..85f7fcd4515 100644
--- a/repos/spack_repo/builtin/packages/boost/package.py
+++ b/repos/spack_repo/builtin/packages/boost/package.py
@@ -748,7 +748,7 @@ def determine_b2_options(self, spec, options):
             cxxflags.append("-DBOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK")
 
         if cxxflags:
-            options.append('cxxflags="{0}"'.format(" ".join(cxxflags)))
+            options.append("cxxflags={0}".format(" ".join(cxxflags)))
 
         # Visibility was added in 1.69.0.
         if spec.satisfies("@1.69.0:"):