diff --git a/lib/ASTGen/CMakeLists.txt b/lib/ASTGen/CMakeLists.txt
index 46d88fef49174..7b9815b919fdd 100644
--- a/lib/ASTGen/CMakeLists.txt
+++ b/lib/ASTGen/CMakeLists.txt
@@ -1,20 +1,21 @@
 add_subdirectory(Sources)
 
-# If requested, build the regular expression parser into the compiler itself.
-if(SWIFT_BUILD_REGEX_PARSER_IN_COMPILER)
-  file(GLOB_RECURSE _COMPILER_REGEX_PARSER_SOURCES
-    "${SWIFT_PATH_TO_STRING_PROCESSING_SOURCE}/Sources/_RegexParser/*.swift")
-  set(COMPILER_REGEX_PARSER_SOURCES)
-  foreach(source ${_COMPILER_REGEX_PARSER_SOURCES})
-    file(TO_CMAKE_PATH "${source}" source)
-    list(APPEND COMPILER_REGEX_PARSER_SOURCES ${source})
-  endforeach()
-  message(STATUS "Using Experimental String Processing library for _CompilerRegexParser (${SWIFT_PATH_TO_STRING_PROCESSING_SOURCE}).")
-
-  add_pure_swift_host_library(_CompilerRegexParser STATIC
-    "${COMPILER_REGEX_PARSER_SOURCES}"
-  )
-else()
-  # Dummy target for dependencies
-  add_custom_target(_CompilerRegexParser)
+if(NOT SWIFT_BUILD_REGEX_PARSER_IN_COMPILER)
+  message(FATAL_ERROR
+    "Unable to build ASTGen without Regex Parser support. "
+    "Please enable SWIFT_BUILD_REGEX_PARSER_IN_COMPILER.")
 endif()
+
+# Build the regular expression parser into the compiler itself.
+file(GLOB_RECURSE _COMPILER_REGEX_PARSER_SOURCES
+  "${SWIFT_PATH_TO_STRING_PROCESSING_SOURCE}/Sources/_RegexParser/*.swift")
+set(COMPILER_REGEX_PARSER_SOURCES)
+foreach(source ${_COMPILER_REGEX_PARSER_SOURCES})
+  file(TO_CMAKE_PATH "${source}" source)
+  list(APPEND COMPILER_REGEX_PARSER_SOURCES ${source})
+endforeach()
+message(STATUS "Using Experimental String Processing library for _CompilerRegexParser (${SWIFT_PATH_TO_STRING_PROCESSING_SOURCE}).")
+
+add_pure_swift_host_library(_CompilerRegexParser STATIC
+  "${COMPILER_REGEX_PARSER_SOURCES}"
+)