diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -919,15 +919,25 @@
 foreach(t ${LLVM_TARGETS_TO_BUILD})
   set( td ${LLVM_MAIN_SRC_DIR}/lib/Target/${t} )
 
-  list(FIND LLVM_ALL_TARGETS ${t} idx)
-  list(FIND LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${t} idy)
-  # At this point, LLVMBUILDTOOL already checked all the targets passed in
-  # LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, so
-  # this test just makes sure that any experimental targets were passed via
+  # Make sure that any experimental targets were passed via
   # LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, not LLVM_TARGETS_TO_BUILD.
-  if( idx LESS 0 AND idy LESS 0 )
-    message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
-      "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
+  # We allow experimental targets that are not in LLVM_ALL_EXPERIMENTAL_TARGETS,
+  # as long as they are passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
+  list(FIND LLVM_ALL_TARGETS ${t} idx_all_targets)
+  list(FIND LLVM_ALL_EXPERIMENTAL_TARGETS ${t} idx_all_experimental_targets)
+  list(FIND LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${t} idx_experimental_to_build)
+
+  if( idx_all_targets LESS 0 AND idx_experimental_to_build LESS 0)
+    if( idx_all_experimental_targets GREATER_EQUAL 0)
+      message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
+        "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
+    else()
+      message(FATAL_ERROR "The target `${t}' is not a default target. It may be "
+        "experimental, if so it must be passed via "
+        "LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.\n"
+        "Default targets: ${LLVM_ALL_TARGETS}\n"
+        "Known experimental targets: ${LLVM_ALL_EXPERIMENTAL_TARGETS}")
+    endif()
   else()
     set(LLVM_ENUM_TARGETS "${LLVM_ENUM_TARGETS}LLVM_TARGET(${t})\n")
   endif()