diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/RegularExpressionDxe.inf b/MdeModulePkg/Universal/RegularExpressionDxe/RegularExpressionDxe.inf
index 883d5f1127..e9c885465d 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/RegularExpressionDxe.inf
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/RegularExpressionDxe.inf
@@ -107,6 +107,9 @@
   # Oniguruma: tag_end in parse_callout_of_name
   GCC:*_*_*_CC_FLAGS = -Wno-error=maybe-uninitialized
 
+  # Oniguruma: implicit conversion from 'UINTN' (aka 'unsigned long long') to 'long'
+  GCC:*_CLANG9_*_CC_FLAGS = -Wno-error=constant-conversion
+
   # Not add -Wno-error=maybe-uninitialized option for XCODE
   # XCODE doesn't know this option
   XCODE:*_*_*_CC_FLAGS =