diff --git a/clang/runtime/CMakeLists.txt b/clang/runtime/CMakeLists.txt
index 26d6c1233cb5..99298e11b1d7 100644
--- a/clang/runtime/CMakeLists.txt
+++ b/clang/runtime/CMakeLists.txt
@@ -27,7 +27,10 @@ set(COMPILER_RT_SRC_ROOT ${LLVM_MAIN_SRC_DIR}/projects/compiler-rt)
 # This is the same behavior (try "internal", then check the LLVM_EXTERNAL_...
 # variable) as in add_llvm_external_project
 if(NOT EXISTS ${COMPILER_RT_SRC_ROOT})
-  set(COMPILER_RT_SRC_ROOT ${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
+  # We don't want to set it if LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR is ""
+  if(${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
+    set(COMPILER_RT_SRC_ROOT ${LLVM_EXTERNAL_COMPILER_RT_SOURCE_DIR})
+  endif()
 endif()
 
 if(LLVM_BUILD_EXTERNAL_COMPILER_RT AND EXISTS ${COMPILER_RT_SRC_ROOT}/)