diff --git a/CMakeLists.txt b/CMakeLists.txt
index 919e5ed8..25845d68 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -451,9 +451,9 @@ target_include_directories(OpenSTA
 
 target_compile_options(OpenSTA
   PRIVATE
-  $<$<CXX_COMPILER_ID:GNU>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security -Wno-deprecated-register>
-  $<$<CXX_COMPILER_ID:AppleClang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security -Wno-register>
-  $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security -Wno-register>
+  $<$<CXX_COMPILER_ID:GNU>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
+  $<$<CXX_COMPILER_ID:AppleClang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
+  $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
   )
 
 # Disable compiler specific extensions like gnu++11.
diff --git a/README.md b/README.md
index 695ed2d1..b43530fb 100644
--- a/README.md
+++ b/README.md
@@ -73,6 +73,9 @@ bison    1.35    3.0.4   3.5
 flex     2.5.4   2.6.4   2.5.35
 ```
 
+Note that flex versions before 2.6.4 contain 'register' declarations that
+are illegal in c++17.
+
 These packages are **optional**:
 
 ```